exhaust leak as in, a hole in your muffler, or like bent exhaust valve/leaking headers? if its just a hole in your muffler or leaky headers, i seriously doubt it has anything to do with slower acceleration. but if its a bent exhaust valve, then thats probably the reason. i bent up my exaust valves(or atleast thats what i beleive happend) when i went offroading a while back, slowed it down pretty bad.
hmm.. i should probably get that fixed.....