I think the most custom pistols you'll find beyond high end gunsmith custom jobs are 1911's and Glocks, just because they are the only two pistols with a huge aftermarket parts market that rivals the AR-15.
I'm putting together my first custom Glock right now. It's a Gen 4 G20 I'm optimizing for protection in bear country (save me the 10mm for bear protection debate please. There are too many factors that go into a bear charge to say anything conclusive, but a round that can go through 14 water jugs in a row and keep going is going to break bones and stop stuff with a precise shot)
Most of the performance parts I've got from Lone Wolf and include:
6" Salt Bath Nitrided Alpha Wolf barrel
LWD pro pick trigger kit
LWD recoil spring assembly and ISMI 22# recoil spring
Trijicon Night Sights
Arredondo +5 mag extensions (won't carry these in the field because they're extra weight and bulk, but they're cool)
I'm debating on a frame mod job from Zev Technologies too. I'll likely go with a stipple job, but I'm debating if I want a trigger guard undercut too. I'm worried moving my hand up will put my fingers on the ridges instead of between them, and I don't think I want the finger grooves removed. I'm also considering a slide cut and a micro red dot that I'll mount for deer hunting.