A longer swingarm is typically used to help keep the front end down under heavy acceleration. I've sold swingarms for bikes with pipes and some porting but only about a +2 or a +4 for more heavily ported bikes. A +6 is more reserved for those bikes that can drag, and dune. Anything over +6 really isn't good for anything but drag bikes.
Most of the shorter swingarms (-1, -2) I've made are for flat tracking, MX, ect. It brings the rear wheels closer to the center of gravity on the bike and allows you to hook up really well. With some decent shocks, you won't wheelie at every turn, you'll actually get some good weight transfer and some squat in the rear end and you'll take off like a bat out of hell.