My G26 has had night sights installed, and I don't plan on changing anything else ...
I have a Glock 3.5# connector I could install, but I won't ... and I've tried NY1 triggers and don't care for them for my weapons. The stock connector and trigger configuration is plenty fine for defensive use, and it's amazing how the stress of some situations decreases some folk's sensitivity and awareness of how "strong or light" trigger pull weights may "feel". A finely tuned 4# trigger is an advantage in a 1911, when used in the hands of a highly trained person who spends a great deal of time engaged in physically challenging training with that trigger ... or in the hands of a very skilled shooter in one of the sporting venues ... but in the hands of most ordinary folks (non-L/E & L/E alike) lighter triggers aren't necessarily an advantage, and may increase the potential for a ND in an unexpected, highly stressful, life threatening situation.
I have a smooth-faced trigger w/trigger bar I could install, but the stock one doesn't bother me at all, even during extended range practice sessions.
The stock magazine springs and captured recoil guide rod & spring has functioned just fine with everything I've used in the pistol so far ... standard pressure 115gr & 115 +P+, 127gr +P+ & 147gr ... and I'll change both the magazine springs and the recoil spring assembly on some reasonable periodic basis.
While I don't see the need to add anything to the grips, some folks might feel they would benefit from some enhancement. As long as what they choose doesn't adversely affect the operation of the pistol ... and I've seen some sleeve-type additions eventually interfere with both the magazine catch and the free insertion/removal of the magazines ... then that's probably more a matter of personal choice.
I think a lot of folks tempt fate by trying to highly modify & "customize" something which doesn't really need it in the first place ... or, trying to "fix" something which isn't yet broken.
I expect my G26 to last a long, long time ... with reasonable maintenance and quality ammunition. It functions perfectly reliably as it is.
Ditto for my G27.
Just my thoughts ... but in many circumstances it's more the user of the pistol that can better benefit from some "upgrades" and "enhancements", especially when it comes to training, than the pistol.