Before anything else, just out of curiosity what settings are your card's switches set to? My HIS R9 290 came with the switch in quiet mode by default (similar to the default setting most 290X's ship with) while many have noted this does nothing on a 290 I've found that the gpu throttled far less in the uber setting than in the quiet setting. Fan noise also went up. I have a feeling that non reference cards in uber mode have alot more change than just a 10% bump to fan setting. My default 3dmark 11 for example went from 10580 with the switch set to quiet, to 12540 with the switch set to uber. I had tried overclocking and setting the fan manually to 100% with msi afterburner, however the gpu still throttle far more in quiet mode than in uber.
Imho your gpu's are throttling and that's what causing the low performance. If your switches on the cards are set to what would be quiet mode on a 290X, switch them to what would be uber.
typical standards from 290X reviews
"Quiet Mode" – BIOS position one. Switch is in position closest to where you plug in your displays. Fan is at 40% cap.
"Uber Mode" – BIOS position two. Switch is in position furthest away to where you plug in your displays. Fan is at 55% cap
if there really is an issue with vram usage as well and drivers don't change it, switching to the other bios may fix that issue also.