That's sorta what I did, 4.1 U1 to 5.0 to 5.1. Most VMs had been upgraded to hardware version 8 but some were still on 7 and even after 5.1 I tried version 9 too. Good news on the tools, at least this way I don't have to worry about stripping out all the tools and reinstalling with the 5.0 version (since I'm not 100% sure which ones upgraded and which ones didn't).
So far, downgrading to 5.0 U1 has solved all of my problems. I do want to see about moving to 5.1 though so I'll probably have to take a spare system and run some sandboxed tests. Fortunately my Precision 690 runs ESXi nicely and has the storage space needed