Add a stainless guide rod and you're good to go. You can do a lot more than that, but honestly Glocks don't need much. Night sights if it doesn't have them already. I wouldn't waste money on any more than that.
I added a 3.5lb trigger connector to replace the 5.5lb stock one, but it wasn't bad at 5.5 .
Sights are the only thing Ive ever changed on all my Glocks. Maybe an after market barrell if you wanna shoot lead. http://www.lonewolfdist.com/ seems to be pretty popular for people wanting to tincker with there Glock.
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I have a preference for the larger calibers myself, but I have a good friend who is a Marine Recon Scout Sniper Iraq vet who carries a Glock 19 in 9mm as his civilian carry firearm. He would take exception with your statement. It's all about shot placement.