Just how much the XP/hr goes down I don't think we'll see unless someone is bored enough to try it like this before it's fixed to require it.
The exp rate even outside of PLing as a whole is messed up, which Yoshi said compounded the problem. I think after the patch 1.19a we can safely expect the rates for ranks 20+ to feel slower.

Notice how he didn't mention anything about a server rollback? All you guys who jumped on it will likely get to keep your precious levels.