Longer barrels DO NOT necessarily improve accuracy, however they DO usually increase velocity. In fact longer barrels flex more and can LOWER accuracy, but at handgun barrel lengths that's not really an issue; it's more of an issue with rifles (longer-barreled rifles are generally less accurate than shorter-barreled rifles; the longer barrel is used to get more range out of the cartridge, not to improve accuracy). It's the increased sight radius, not the longer barrel, that causes people to shoot longer-barreled guns more accurately than shorter-barreled guns.
And I'd be careful about adding some of those parts you mentioned; you might find out you like them less than the original parts. One of the things I like about the Glock is its small slide stop. I use a thumbs-forward grip and it's less likely to get under my thumb while shooting and cause the slide to not lock open on the last shot. In fact, I wish the slide stop was even smaller. I'm a firm believer in using the slide "slingshot" method instead of the slide stop when reloading, so on most guns all the slide stop does is get in my way when shooting. The only thing I use the slide stop for is to lock the slide open manually when clearing the gun.
0331: "Accuracy by volume."