I have a rig with one RX 550 4 GB, one HD 7950 3 GB and one R9 290 4 GB. All mine ETC 59 MH/s @ 398 W at the wall (measured with power meter).
Also it's an 850 W PSU, only bronze rated.
RX 550: 11.35 MH/s
HD7950: 24.46 MH/s
R9 290: 23.13 MH/s
I'm trying to buy older AMD cards because they are really cheap and still profitable, if you known how to config them.
Weird that HD7950 is higher than R9 290, i think it should be close to 28-30Mh/s