winchster,
My advice to you would be to get what you like the looks of as there's not alot of difference in performance between all the different systems. You can play with different baffles to obtain the "right" sound for you, but consider the shorter the pipe the louder and harsher the sound will be. These bikes are not performance machines and trying to squeeze maximum power from one will drive you crazy as well as to the poorhouse! A 2 into 1 system would perform better than a 2 into 2, but nobody I know of markets one for a 750 Spirit.
To get the most out of your engine, either add a freer flowing intake or do the "de-snorkleing" with a K&N filter and a Stage 3 jet kit. These mods, along with an aftermarket exhaust will wake up the 750. Beyond this, it'd be cheaper to trade up to a larger bike. You can spend up to $3000 to gain the same horsepower rating of the 1100, which weighs less than 50 lbs. more than the 750.
Brian