rx 470 4gb red devil (27ps)... mod bios... 1080/1950-1080/1975 27,9-28.3 mh/s ETH/ETC and about 275 sol/s on 1140/1900 ZEC/ZCL

You could do better by bumping the core clock somewhat - it's too low.

Tried it. 28.6 MH/s, but power consumption is growing strongly.

Yeah, I have my cards at 1150 (linux) for 31MH/s. It's a good balance, IMO.

Which model / brand do you have?