For ETH mining then yes it makes a difference because it runs like crap on stock bios.
Stilt bios for the 290 is only for the reference models.
Could You tell me what is the biggest difference made by it? I have water cooled all my reference cards (temps. 69-78°C on the core, VRM still hot 90-115°C), hashrate on Ethereum 30Mh/s and wattage 270-280w from the wall. I would be happy if I could get my cards eat up a little bit less electricity and cool down the VRM temps., other than that I am pretty happy with the current results.
Edit: I will add bigger heatsinks to my VRMs, trying to get the temps below 90°C.