C8 has some improvements in the vertex modeler and I think you will like those.  There are also a lot of cool gizmos that have been added that is always just a good thing (can't ever feel bad about gizmos!).

You are correct to point out however that 90% of the changes which have been done to C in the last three versions are focused on POSER/DAZ and their related animation developments (those are very good and important things to be sure but not if you're just a modeler/renderer).

In terms of rendering, here's my experience.

C8 will certainly be faster.  In terms of GI, it's a whole TON faster in the precalculation sequences (like body slam faster).

When it comes to the point where you actually crunch the normal render, I don't find it a huge improvement (but it is improved).  So if you're the kind of person who does a lot of GI types of renders, then this will be a significant improvement.  If you're not, it might be a smaller improvement.

C5 is a long time ago so I can't remember all the changes since then.  There have also been some nice upgrades to the texture engine since then and you will probably like those.

Oh yeah and by the way, "body slam" faster is FAST!

-Kix