It looks like you currently have a peak turbine flow of about 42lb/min. The GT4202R w/ 1.15 a/r matches that turbine flow nicely. I would think that you would have close to the same response with the single with equal turbine flow. You are loosing out on the divided turbine housing with this new manifold. Your twins took advantage of having cylinders 1,2,3 and 4,5,6 divorced. This theoretically helps spool up.
Justin