It sounds as if you have a clogged exhaust. The cheapest fix would be to clean out the exhaust. Refer to Fred's Guide in the resource section to see how to clean out the exhaust.
Or, you could replace the exhaust with an after market performance exhaust, and re-jet the carburator. This will increase the performance beyond what can be achieved with a properly working stock muffler.
The Airsail kit will be relatively useless unless you fix the exhaust. However, since the Raptor kit includes a new exhaust it would fix the problem as well as seriously boosting the performance of the bike.