My observation on 2x  r9 280x on multipool.
hashrate in sgminer up 35%, el. consumption up from 265w to 300w. Payouts around 5-10% higher (not enough testing, to be sure if it's any gain), temps 5 degrees higher.

summary:
sgminer shows big increase of hashrate, power consumption higher 10-15%, payouts not even close to 30% higher, probably around of higher power consumption mark, max to 10%.

And my data is with disabled 2% fees (using: P C 0 ).