The hashing rate could be decreased from 1 per second to, eg, 1 per 10 seconds. This would be an easy way to decrease the CPU usage (assuming it comes mostly from hashing). This would likely increase the variance of staking times though.
Elsewhere, I also read that you'd like to speed up syncing. Old blocks will still have to be verified using the existing method, so any speedup will show gradually as the "new style" blocks increase in proportion to total blocks in the chain. So a change will not mean faster syncing, but merely a lesser increase as times goes.
Last, someone mentioned ASIC friendly. I doubt anyone will stake HYP on an ASIC, so I would not think this to be an interesting property for deciding on a replacement.
In the end I don't really have a theoretical preference for which hash to use. I'd favor a simple one for which good code already is there. And since you said SHA256 is already used for coinstake...