At only 6 channels in currently, that would be just 450 of the 900MIPS available - hopefully the remaining 450MIPS is good enough for MRC and 6->8 channel expansion.
Unfortunately, it doesn't quite work like that. Until the G68 "morphed" into a 900 MIPS machine from 750 MIPS, it was believed that 150 MIPS was available for upsampling on the receiver card with the other 600 MIPS reserved for DSP presets, time and level alignment, and MRC. So is the split now 300 MIPS and 600 MIPS? Not clear at all. Also, whilst the 808.2 uses a 150 MIPS chip, I doubt that all of it is used up.