the idea that you would lose power due to an oversized exhaust only applies to naturally aspirated cars that rely on cylinder scavenging to draw exhaust out of the cylinders. with a turbo in place, after the turbine wheel starts spinning it creates a vacuum to draw out gasses, but usually pressurized gas WANTS to get out of the cylinder anyway, just as any pressurized anything wants to expand.
turbos "spool up" faster when there is a high pressure differential between pre turbo exhaust and post turbo exhaust. a "low" pressure in the downpipe and a "high" pressure in the exhaust manifold will cause the turbo to spool quickly, while a "low(er)" pressure from an aftermarket custom manifold and a "high(er)" pressure from a too small or stock sized exhaust will cause slow turbo spool.
so to sum up, N/A cars have to worry about exhaust size, turbo cars, go as big as you can, you really won't lose anything. that being said, space pretty much confines an alltrac to 3" pipe. so go 3" if you can. you'll spool faster and will probably make more power than 2.5", but if it's done then it's done don't do it again till you need to.
