Dunno about drivers, for me when I tested 16.4.1 crimson they benched lower than 16.3.2 WHQL so stuck with them as not having an issue in games, etc I use. Yet to be bothered to test 16.5.x .
I'd think 2 aspects come into play with increased HBM clock.
i) RAM having errors as can't do the frequency.
ii) Memory controllers can't handle increased frequency of HBM (8 x 512-bit memory controllers, 2 per HBM stack).
Both of above I'd say is and/or situation.
Mumak (author of HWiNFO) recently enabled EDC counter for Hawaii and below AMD GPUs. Now members are seeing that Hawaii MC can't handle as high a RAM OC error free as they can attain artifact free. Mumak has tested EDC counter on Fiji but causes BSOD, perhaps he will suss it in future release.
See this post
on EDC by The Stilt in the context of Hawaii/Grenada.Edited by gupsterg - 6/1/16 at 9:46am