What performance results would fixing these symptoms give for my car?

So my car is an 2003 Honda Element 4 speed auto awd. Sometimes when I take off the line my truck accelerates quicker than it does at other times meaning sometimes when I takeoff from a light stop, it gets up to high speeds quicker at lower rpms than usually but that is usually after I've driven the car long enough to really warm up. Also it gets up to higher speeds more easily. Initially, after a few min of warm up, it takes a bit of effort to get it accelerating off the line.
My car has 140,000 miles btw. Use 6-20w synthetic oil by royal purple

Anyways, I recently had a Diagnostic test done on my car due to a check engine light and they said I need the following:

-a new upstream o2 sensor
- fuel induction service done(never been done on my car)
-new set of NGK iridium spark plugs(OEM)
- and I might need a new engine air filter as well(not part of the diagnostic but it has never been checked in a long time).

Based on these symptoms, if I resolve all these problems, what performance gains will I get in terms of acceleration? Will it be slightly and noticeably more quicker and responsive?

The cheapest, simplest and most effective improvement would have been to replace the air filter which you stated has been ignored. I can almost promise you the oxygen sensor was not faulty, just the idiot doing the diagnosis.
Also, why haven't you stated the code number (s)?

Only slightly noticeable acceleration increase. Smoother. Better mpg.

Honda Element and performance is a wet dream you're having jojo. Just drive the damn thing.