Just bought a set of Edelbrock RPM performers with stage three porting from Hughes (will flow about 325 cfm). Was running ported 915s flowing at 281 cfm that had milled down through the years without compensating on the intake side of the head. Always had intake manifold fitting issues and did not want to cut mill my intake down.
I estimate by the cylinder pressure I was about 11.5 -1 on the 915s.
Dave at Hughes told me I would have to go about 13-1 with the aluminum to really see the benifit of the increased cfms. We ended up milling down the heads and the intake side of the head for proper fit of the intake, am also going with 0 deck on the Ross flat tops. All this is going into the new World/ MP performance block. Am really excited to get this together and see how she flys.