the biturbo is going to perform better on a stock cylinder. Also, youre going to have to upjet regardless of which pipe you choose. And changing the rear sprocket does not change the RPMs the engine is producing.
I just upgraded from a biturbo to a treats pipe on my kitted A55. Ran the biturbo before I kitted and it produced a good top speed, albeit the low-mid range performance was less then ideal. However, that is the trade off you make on a stock cylinder when shooting for a high top speed.