Hi,
just for information - does not have exact your situation but -
I'm using VS2013withUpd5 and some PostR3CU9 binaries (don't know exactly but something after CU9) and this seems to work like a charme (at least on a 64bit OS, 32bit tools can only be installed starting with CU10), but never tested CUS only VAR and USP.
One Question : Did you directly start with CU8 (or did you have some R3RTM (or R2) 'things' on your machine?
Did you deinstall the old VS2010 tools?
I remember some problems during installation. And the solution was always, deinstall the client things and reinstall from a slipstreamed dvd. Beside you already did the direct way.
Do you have EXACTLY the CU8 kernel (the one from the package)?
Maybe I can check this next week, somewhere having this constellation (but with Vs2013Upd5). So let's see.
Did you (just for fun) checked USP layer?
regards
Douglas