I already tested power consumption with both power limit and undervolt. If you already have an R290X you can do it yourself. Boot up Valley or whatever bench you use, leave it on auto and bench away. GPU-Z monitors VRM data on current flow at a certain voltage, converted into watts, that's what your GPU actually use, minus any inefficiencies in the system/PSU (readings from the wall).
Then its just a simple case of testing it with -25% power limit in CC and -50% and see your clock rate & performance. Mine drops from ~175W avg Valley usage at stock to ~105W. Peak power drop from ~226W (which is equivalent to ~250W from wall), to ~170W. Performance loss is 11%. My GPU under load maintains its fan at idle speed 20%, so I know for a fact the power usage definitely drop heaps due to the lesser heat output.
Info here:
http://forums.anandtech.com/showpost.php?p=37596583&postcount=136
Basically I can get an R290X performing like a 970, at even better perf/w. That's the nice thing with undervolting control.