You adjust the idle by the large inset brass screw at back of the throttle housing. You'll need an angled screwdriver, and it only takes a very slight turn to adjust. Idle speed tends to increase as the engine gets run in and loosened up. Less than a quarter turn put mine right and steady after a couple of thousand miles.
Backfiring on after market exhausts is normal and doesn't necessarily signify a leak. And yes, it revs higher and easier without that gawd awful stock torpedo tube of a silencer blocking up the works.