Just to clarify things...That is only half of the story, HR also makes timing windows tighter than OD10's by a decent about:
With hard rock enabled, any diff with OD8 and HP8 will be adjusted to OD10 and HP10, which is easily done, despite with a huge accuracy drop from players. The HR gives score boost that isn't quiet necessary. Reduce the score multiplier to 1.05 would be just fine.
This is OD 10 nomod
This is OD 10 with HR