My unvoltaged and underclocked 7970 uses about 140W to have 20MH/s. So that is less efficient than the 970.

I think it is similar efficiency. the difference is very small. 7970 is 3-4 year's old card.

7970 20MH/140 = 0.142.
970: 20.75/147 = 0.141.

i have almost 22(21.6 average) at 150w-160w(tdp 60%)

22/155 = 0.141. So Your 970 (?) is the same efficiency as the above AMD and nVida.

I am wondering how good is the 970 after undervolting.

yeah same thing, in the end, with my 970, but better than old card