It should be noted that I have some reservations, and I'd like to discuss them in greater detail as to why I'm hesitant to add anything like this.
1) Weapon statistics are not intended to perfectly model the differences between all models of a particular brand of weapon. In game terms there is not a huge difference between weapons in the same categories. Weapons can for the most part appear as you describe them. An oversized meat cleaver and longsword may very well share statistics in game terms. With more modern weapons, there is not enough difference between a German Luger and a Beretta M9 to give them different damages, and so forth. I feel that this is seriously pushing towards micromanagement in a direction I have no interest in going, and over-complicating the game.
1.a) I will freely admit that there is some disconnect when it comes to modern weapons, which firearms border on already. Alvena has cylinder firearm technology, which means revolvers and revolver-rifles are available. I understand that guns and such have many modifications and design differences in modern day. Even things such as how many shots a gun holds can vary from gun to gun, as well as the caliber of the round, and lots of other minor details like that. The fact of the matter is that guns in Alvena are simply not that diverse yet, nor are they intended to be. I think it's great that Alvena not only has a place for guns, but that it doesn't keep them ferreted away solely in a single region where people can't get them (as is customary for both Forgotten Realms and Pathfinder's Golarion setting), but at the same time you're not going to be seeing a wide range of different firearms in the setting. Some might look different, have different artisan logos, and other features that set them apart in appearance but they are all mechanically similar.
It's possible that an expanded range of firearms could be added (d20 modern for example has a ton of different makes, models, and styles of handgun alone), but I frankly have little desire to build a better gun at the moment. Alvena firearms already offer pretty much all anyone could ask for beyond burst or automatic fire which are not currently welcome in the game. They are available and trade-able at almost any town, can be used by almost any class, can be specialized in fairly easily, have benefits for martial and magical classes, and come in varieties that allow you to shoot up to five times before needing to reload (which means even with a perfect BAB and haste you can fire until you run out of attacks). For high levels we even have magic cylinders that refill themselves for a mere 3,000 gp. Our firearms sport good distances and damage, especially for one-handed weapons. The two handed firearms are about as powerful as anyone could reasonable expect (shotgun weapons average 10.5 base damage, and rifles average 7 base damage at long ranges, and your average person only has about 3 hit points).
2) I have no interest in adding any sorts of systems or subsystems that involve changing the statistics of a weapon based on modification. I've been down that road before and it generally does not have any real benefits. Mechanical representation means that there will be a clearly superior way to optimize a weapon. It happened in Star Wars d20 (when the Arms and Equipment guide was released, everyone who knew anything about optimization took a -3 penalty to damage for a +3 bonus to hit because they understood math, or happily added +6 to their stun save DCs), it's happened in other d20/D&D based subsystems, and I'd rather not have it happen here.
3) If I was going to consider a subsystem that would allow characters to modify their weapons, it would more likely come at tradeoffs that cost more than gold (such as +1 to hit but -2 to damage). More than likely I would end up revising the way equipment was handled on a grand scheme, and it would affect the way magic items are handled. I may end up doing something like this in the future as part of an effort to raise the usefulness of the Craft skill (as I have been considering creating options for action-RPG style modifications like Diablo II / Torchlight I & II style gem socketing, with the option to create or add sockets or special features to items), but such things are not planned in the near future if they are to happen at all.
So as we all know, there are pretty much a bajillion ways to upgrade/enchance equipment through magical means, but there are hardly any non-magical ways to enhance or upgrade equipment, which really doesn't make sense for a universe steeped in magical prowess, filled with intelligent super-beings and gods all around to not have any non-magic reliant technology.
I actually disagree with this heavily. If magic existed as it does in D&D, virtually all technology in reality would revolve around it to this very day. Magic is literally the solution to energy problems, famine problems, weapon technologies problems, and more. It is literally the power of the gods at your fingertips. Humans are very prone to getting comfortable with stuff that works. We are actually in a bit of a problem right now because we got really comfortable in reality on our reliance of fossil fuels and now their decreased availability and hazardous effects on the environment has us scrambling to look for alternative means of energy. It's actually a wonder at all that firearms even exist in a fantasy setting where magic is available. Things like tanks run by steam and magically produced fire, airships, and stuff like that I could easily see. Taking the time to trial and error through the creation of a firearm that functions and functions well in a world where people can throw lightning bolts or lob angry earth elementals from catapults is kind of a fantastic stretch all of its own. To get an idea as to what I mean, check out the tv series Avatar the Last Airbender. There is basically an entire nation of people who can use "firebending" which is effectively fire magic on demand. They are renowned smiths, engineers, and create things like huge iron ships, airships, and even tanks that can drive even if flipped upside down; but they don't have guns because they literally just lob destructive explosive blasts of fire at everything; so guns have never been invented in their world (one of the characters does get an adamantine sword though).
That being said, it does bother me that magic is the ONLY method of advancement. Which is basically why I've been considering adding some method to improve on weapons using mundane means (as mentioned above). However it is not something that I would suggest planning around at the moment. What I can suggest planning around what currently exists, work on your character concepts, fluff your weapons to look as you desire, make use of what you have. Play the game a little. Stop and smell the flowers. There is nothing here in this thread that has any real bearing on your character, their motivations, their flavor, or would make any meaningful impact on how your character is perceived by others.