Misanthropist just explained why somone would do this mod. It makes the gun point a different way out of the holster than it normally would. Glocks tend to point naturally high for me, so i use a GFA to help alter grip angle. It does help with first shot speed out of the holster if the sights are actually somewhat aligned when you complete your press out, instead of the front sight pointing high.
Pretty common. Can you train to it? Absolutely, and in general, it IS the shooter not the grip. You can grip all kinds of shapes effectively: a hockey stick, a golf club, a tennis ball, or a K-frame or a Mateba or a Glock and if you ONLY use that one pistol you can train your body to maximize that shape. When I only shot Glocks, which I did for a couple of years, I didn't find a significant problem with the grip angle, I just trained to that angle.
If you switch platforms, you'll find that nothing other than a Luger has that angle. It's too shallow. The M&P, the 1911, the 226, the P30, the USP, the HK45, the CZ75, the BHP...they're all pretty similar and the Glock is out in left field.
If you are someone who likes a couple of different guns AND you want to maximize your performance with both of them, changing the grip angle to be more congruent with your other gun might not be a bad idea.
As far as the grip circumference goes, I haven't found it to be an issue and I have huge hands. I used to keep an Ipad in an Otterbox for work, and jokingly pull it out like a phone, holding it in one hand like you would hold a regular cell phone. My hands are big.
It's probably not a problem simply because the main area you're removing material is at the bottom of the grip, where, in my experience, most people have less contact than anywhere else. A smaller circumference doesn't really hurt you there.
Texture is kind of a personal thing and some people will be adequately served by the Gen 4 texture, which IS a big improvement over the Gen 3 and below. I stippled one of my Glocks after slipping on a range in a monsoon-grade downpour, and getting my right hand muddy, then having to go for my gun. The gun was slippery in my hand with the really soaking muddy texture and I thought I could improve it. Turns out I could and it wasn't very difficult. The gun is a keeper, so why not? Besides which, it was a used $400 Glock when I bought it...who cares? Zero collector value.
If the OP gets a performance improvement off the work, awesome. If not, well, it's just another Glock. They'll make more.