Gain looks to be ~0.7-0.8Mh/s (ethash) per card, or about 2-2.5%
(Polaris)
What is the power increase for that?
The Claymore boost increases exponentially the power consumption for similar extra hash, which is not effective...
Nothing - basically increasing REF should just be replacing refresh reads/writes with workload reads/writes, so you really shouldn't see any power use change, unless your RFC was so high that you have a bunch of NOOPs during the refresh period.
FYI, my power use is ~130w for 32.25Mh/s per card avg (which seems a bit high to me, given that i'm only at 1175 cclock, 2100-2200 mclock, and 837mv avg, tho it's a little warmer today, so I think my fans are pulling a bit more)