For the new Gen 4 Glock .40 Model 27, I performed a few modifications for improved handling:
1. Substituted a smooth-faced trigger for the standard target-style serrated trigger. Smooth triggers are more suitable for fast defensive shooting.
2. Substituted a NY-1 leaf-style trigger module for the standard coil trigger spring. Some folks say that the former is less prone to breakage than the latter, but breakage is irrelevant. The Glock's beauty is that it'll still cycle when the trigger spring breaks until the mag runs dry. Emergency reloads without a trigger spring should number amongst one's malfunction drills.
The NY-1 trigger spring provides a revolver-like feel and increases the pull weight by a few pounds. I'm fond of sticking guns in belly bands, and the standard Glock trigger's short travel after take-up makes me nervous. I'm probably the only person in the world who prefers the NY-1's feel over the standard set-up.
3. Substituted a magazine extension for the standard floor plate. I've tried both Glock's and Pearce's extensions, and I like Glock's better. Both work fine.
And, yes, I am qualified to perform modifications.