r9 290/390 give 29-30 mh/s while mining on Ubuntu. For a serious increase in performance, you have to use Ubuntu version no higher than 16.04 (with new Ubuntu kernels these cards have poor compatibility). You have to install driver version 17.40 and AMD SDK kit. Switching to Linux-like OS gives additional 10 mh/s (50%). There is a guide how to do it (in Russian): https://www.cryptoprofi.info/?p=3313. Ubuntu will be useful for mining on all old cards including R9 280, 290, 295 and 390.