Since they are going turbo I'm willing to bet they will try to limit engine size as much as possible. Probably stay with 3L engine. This will keep gas mileage up and provide thicker cylinder walls than would 3.2L give. All this to support higher boost levels.
Also if you look at previous models; M3s were always within 17-20% of M5s..
This puts us in 450-460hp as most rumors were going.
2000 M5, 161k, Ronin edition
2011 Cannondale Caad 9-5, 52mph top speed