Still inside your massive ranged 30-50% guess, that's quite a difference and as said the 290 was only 20-30% better than a 7970, not 30% in everything.
It could also be 6 months away, by that time why not just wait another 6 months for the next best card? Your arguments are pretty much flawed by a lot of guessing and waiting around.
If you're going to compare the 390 to a 290 then the 970 should be compared with a 770 on those grounds as they are both "mid ranged" remember and it's the same 20-30% between a 770 and a 970. Just the same way a 980 is around 20-30% better than a 780, and the 980ti will likely be around the same compared to a 780ti.