I would be surprised to see you break 250. Dont take that the wrong way; I would like to see your setup prove us wrong.... I just don't see it happening.
The theoretical limit of the M62 blower spinning at 14k rpm will push around 450cfm which could make around 300hp on average. It takes about 35hp to drive the supercharger at 14k rpm. That brings the aprox. max power output to around 265 before any drive-train loss.