Do you think the smaller overall diameter makes it a harsher ride? Or is it gearing/speedo error that worries you?
I think you'll definitely get tyre rub with 70 series tyres. My 60 series are fine but when I tried my friend 65 series tyres it rubbed the rear outside tyre edge badly on the kick panel at full lock.
HBs definitely don't seem to have as much room between the edge of the wings and the outer tyres compared to HCs from what I have seen.