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.
I will give Ubuntu a try for these old cards. But how about rx 470 / rx 570? Will they also have higher hash rates with Ubuntu?
not just linux, it's the AMDGPU-Pro driver improving things. I was testing an old HD 7850, gets 10-13MH/s on windows and FGLRX on linux, but AMDGPU-Pro on SMOS got me 17-17.7MH/s. HiveOS just refused to use the 7850. SMOS and HiveOS both got me about 27-30MH/s for an R9 290, but only 19-20MH/s on windows. My RX cards were the same on linux distros in general. The RX Cards were the same on linux and windows with compute mode enabled*. linux was doing the same thing as the compute-mode by default, I believe AMDGPU-Pro is basically defaulting to compute-mode behavior for all GCN cards while windows only uses that behavior on GCN3 and up.