well yes and NO
yes a 2.5 inch group will for sure kill a deer, just think many guys shot much bigger groups at say 30-40-50-60 yrds and kill deer with them no problems(and not starting a distance war either)
but at 20 yards with a scoped X box, that you KNOW can do better(as in the past it shot better) I would TRY to see if I could improve on it, IF I really wanted too, or just doubl;e check to make sure all is well with the gear end of things!
I know you limit your shots so as is!, will for sure work for you!
arrows do wear out you know same as all parts on things, could be a string going a little, could be lots of little things
they do sell scopes with more maginfacation than can sure help aging eye's to see a little better at distances, I know my vision isn't what it once was too, and I like good glass on my weapons anymore!
most triggers can be tuned up I would think, one of the reasn some higher end X bow's/gun come with them!
limbs do get old and loose there repeatability, equal power side to side and all sorts of things, there like people, they get old and don't work as well as new!

easy way to tell if its equipment or shooter is go to a few bow shops and well test fire a few now models
not saying you need to buy one, but just to see if its the shooter or gear, shooting indoor help eliminat some things too!
you can also have a different person shot your same set up and see how that goes, if they shoot better than you, then you know its ONLY you and not the gear being the issue!, and can make adjustments from there , like again a new more powerful scope or trigger job or??

me I am normally a nut,on over doing replacing things before really needed! I do a new string on my X bow every yr, for peace of mind, there only like 20 bucks so to me its cheap insurance one didn't stretch over the storage of off season
and when it comes to rifles, I am an accuracy nut of sorts, most of all my hunting rifles shoot sub half inch, MOA
again its not needed, but sure add confidence at times
or maybe its from me being a long range shooter, I like small groups and the challange to get them the farther the more challange!