Most polymer guns do not lend themselves to "customization".
Glocks do lend themselves to customization, and they're actually easier to customize than most other handguns. That said, grip reduction/reshaping and better sights are usually all you need.
Here's a G23 with Heine two-dot night sights. Robar has reduced the grip size, bringing the grip angle a bit closer to that of a 1911 and shortening the trigger reach slightly. They removed the finger grooves on the front strap and extended the backstrap below the magazine, then applied their medium texture finish to all modified surfaces to replace the checkering. Everything else is stock.
This is G19 with three-dot Trijicon night sights. I relieved the trigger guard and cut the the finger grooves off of the front strap with a rasp. This took about ten minutes. No further work has been done, but it will go out for grip reduction soon and I may remove the hook from the front of the trigger guard. Everything else is stock.
This 15-yard group is right at 1.5", or about the size of a human eye socket. I made it about three hours ago, using Winchester 147-grain factory ammo and the G19 shown above, which has had no accuracy work done. As with any firearm, the key to shooting the Glock well is mastering its trigger. Most gun people (including me) hate Glock triggers. It took me 1,000 rounds to learn the trigger, but now I shoot it decently. I also shoot my other guns better as a result of having learned the Glock trigger. No, it's not what we're used to, but it can be used well if you're willing to learn something new.
To shoot a standard Glock well, I have to align the center of the backstrap with my lifeline, which is not as secure as I'd like. It's OK for 9mm recoil, but it's uncomfortable with the 40. The reduced frame is more comfortable and easier to grip properly. The ideal combination is the G19 top end on the reduced frame. It's faster from a holster, it's accurate, and it functions 100%.
The plastic factory sights are horrible. They break easily, and I find the big dot and the rear outline make them hard to align while offering nothing in low light. (I train a lot at night—that's when most gunfights happen.) Midway sells steel Trijicon night sights for $85; they fix both of these problems.
So this is what I've learned in two months of Glock ownership. On the down side, they have all the soul of a staplegun, and that diminishes pride of ownership a LOT. I won't pollute anyone's tender ears by praising them beyond showing what I've learned. But I should point out that the unit once known as 1st Special Forces Operational Detachment-Delta now issues the G22 and G34, except to a few of their people who prefer the 1911. And it would be one-sided to ignore the fact that some regular Special Forces units issue the G19 to their people in Afghanistan. The people in these units are no-sh*t gunfighters who face fanatical opponents every day and win. They have their choice of weapons and they don't have a bunch of lawsuit-fearing lawyers forcing them to make unrealistic choices. We might all set aside our prejudices and note their choice of weapons. I did, and I'm glad.
Hope this helps,
Okie John