Speccing turbos for bikes is quite difficult. They normally have a totally different ration between the hot and coold sideds to a car.

With a 1000cc four pot. You ight want to flow the same 300 bhp worth of air as a car but you have much less inertia from 250 cc slugs of exhaust to spin up the turbo.
Not a problem with your 900cc cylinders.

I take it this bike is a Harley Vrod or something?