Correct me if im wrong here but this is how i look at it.
7990 = 2x Non ghz 7970.
7970 ghz = 5-15% > 7970 non ghz = 290x stock is roughly 43% faster than 7970 stock(using your 37.5% as base).
With some luck we may be able to drag atleast 10% out of that monster putting us in the 80%ish faster bracket vs stock 7970.
example game1
7970= 100 fps
290x=143 fps = 43% faster vs stock 7970
290x 10%oc=157,3fps(with perfect performance scaling that gives an extra 14,3 fps)
7990=195 fps(near perfect scaling which isnt always the case)
(100/195)*157,3= 80.6=19,4% slower than 7990@stock
And this example is even set up to be showing worse case scenarios with the 290x as i believe it would be possible to get more than just a 10% OC on it.