No, I don't think you can undervolt the 290's. Im getting 880-890 per card. Also what is WU?
I undervolted mine. WU is Work Utility, it's what counts really. KH/s is nothing if you're getting low WU.How do you undervolt with a r9 290? My card has locked voltages. Personally I try to keep the temp just below 80, and I run with engine clock at 980 and memory clock at the stock 1250 (upping memory clock doesn't do much for me). Hashrate is at 865-870kh/s which I am satisfied with. 
I undervolted using MSI Afterburner, mine weren't voltage locked.You can up the fan speed to 85%, but don't go above that.
Why?
I put my fan at 70% and out of 10 I have 3 not working after 2 months..