the spacers dont have to be large to run 2.5". i just ran 3/8" nuts as spacers on cars i ran larger pipe on.

you could even run 2.5" back to the brace and then go to 2.25" through it. that would give a little more room for the gasses to cool and contract before going to the smaller pipe. i did something similar over the axle on a roadrunner once... 3" down to 2.5" for the u-bend, and then back to 3"(for the look).

large exhaust increases power by moving the peak up in the rpm range, but this also reduces low end power a bit. a modified engine often should have larger pipe, but a stock engine does not usually need it or necessarily want it. exhaust flow will also be a factor in optimum carb tuning. (very important on motorcycles, not so much on cars)

if 'performance' for you is top end, or you are on the highway alot, go with 2.5". if you prefer low end, go with 2.25" unless you have a larger than stock cam and high flow heads. (IMO) the difference may not be very noticeable other than sound, dyno runs, and race times.