I assume you're talking about client-side performance? If so, then yes it should be much better in RHEL6.
FWIW, The NFS tools don't really do much once you've got the filesystem mounted. (Ok, that's not 100% true with NFSv4 or Kerberized NFS but it is for the most part). Performance problems like this are generally in the kernel. I've got a patchset queued up to try and improve NFS write performance in RHEL5, but it probably won't go in until 5.7.
See this page if you want to test out what I have queued up so far. It's still pretty rough, but the results so far are quite promising:
If you have a support contract, you should open a support case on this. Typically, performance problems are all about numbers so if you can quantify the problems you're seeing then we should be able to help.