2.5" is what should be used for the amount of HP going out... it really just depends on what else you plan on doing. Unless you plan on REALLY opening up the exhaust.. headers, no cats, and maybe keeping mufflers, you dont want to go any smaller. But even thats iffy.
I havent heard the dual JBA set up, but didnt care for the single. Ive also heard they hang lower so you dont have to arrange for the bumper cut out. I dont like that idea at all.
Ive also heard there've been some shipping problems with things backordered and comming in weeks later.