I would say 115's on a 21 butt is over powered. Weight of the motors is one thing, over poweredd another. Is your 21 a centre of the cabin version. Most of the cabin ones I have seen have had 70-90hp. When John butt built ours, he tsaid we mad to put 85's on. his recomended for cc is 2x60's. I do know the odd guy that has 115's on their 21 cc butt, and all they get extra is less $$ in pocket on fuel. Sure a bit more speen but in the same conditions witha bit of a chop, you cant really go faster than another boat without breaking yourself, your crew, your boat or all of the above.
If you looking at changing due to consumtion, rather look at 90 4's. will definately bring down consumption.