Do the 12 hole mod and jet up accordingly and there will be a hp gain. You may as well go with a better airfilter while you are at it. That way you will only have to rejet once. Alot of people like the TwinAir Powerflow kit. Add a clutch spring ( I like the Dalton Orange/Blue ) and would will definelty notice a difference in performance.
Pretty much any after market set up is going to give you a little performance as the stock 2 in 1 is very restrictive---If you look closely at most of the dual header pipes they step to a larger ID about a foot away from the head---Then some step up again before going in the muffler---Meaning more air flow---Not so with the Yoshi---Their header pipes are pretty close to stock all the way---So my guess is Yoshi's would probably be the most restrictive of the duals---And probably the least HP gain.
Way more bark than bite, One of my friends had one on his stock V, after I put the Muzzy on mine I was easily walking away from him. After the 12 hole mod twin air filter and changing clutch weights, Epi Spacer and Or/Bl spring it wasnt even a close race.