I want to do some overclock to the hynix, which are the 8 cards pulling 15mh/s , so to get 18 or 20 mh/s is it possible?

You should be able to get 18-19 from the Hynix overclocking the memory.
Raise the memory clock to 9100 MHz and share the results with us.

Should i play with the power ratio?

With power you should play with [-] put it at 65 or 70%
BTW what OS/mining software are you using?