Should be close to the same but you would have to measure the 906s to be sure. If the 906s were milled you might loose compression going to the RPM.
Big difference in flows in ported 906 heads. Have you any flow figures for the ported 906 and what a RPM does on the same bench? A good 906 could outflow an OOTB RPM. Then you would gain nothing but the weight savings unless the RPMs were ported.
