I did the move and also looked at several source RPMs. One thing to note is that by default, you'll end up with packages replaced by updated packages from UEK repository. I've removed the UEK repo and replaced all packages with the corresponding base packages. That brings you very close to what you have with RHEL/CentOS. Additionally what I found in the source RPMs is that Oracle decided to add some patches/changes fixings issues tracked in Oracle tracking system.
I don't think these changes are a problem because they mostly fix things which maybe RedHat voted not to fix. At least that's my impression.
If you mean that Oracle has patched the base packages, then surely it is then no longer a RHEL clone. The whole point of CentOS was that it was a RHEL clone, warts and all. In that context it doesn't matter how close you get to RHEL, it still isn't the same.
P.