Poser 7 and 8 can work off the same drive; upon installation, different folders are created. I have copies of P7, PoserPro, and P8 coexisting without problems.  Why not just create external runtimes.  It will remedy the issue of 'porting' files from one version to the other.  Yes, P8 is faster but it is not the incredible zippy rendering you are anticipating.