I am going through this very subject on a bird I own, the H9 Cap 232 33%, first flew her at 24 3/4 lb, changed out the NiMh batteries to Li-Ion packs and now working on CF goodies to go with it, she is currently down to 24 and I can tell a difference in its abilities.
However as mentioned it is not much, but I think a lighter bird (within reason, not to mention do we really want to take a $450 ARF and turn it into a $640 ARF) does the high alpha stuff much better, which is what I am learning.
I say fly what ya got and dont worry about it!!!!!!!!!!!! A day of flying will always be better than a day of work, unless you get paid to fly! LoL.