Going from a 4GB 5970 (no reduced clocks, so same as a pair of 2GB 5870's) to a 7970, I definitely noticed some performance drops, though not in all games.
As a whole, though, I'd say any game that was playable with the 5970 was also playable with a single 7970. If I had to choose between the two, it'd be a single 7970, if for no other reason than that there are some games which don't handle multi-GPU setups properly.
In this case, however, we're talking about two 5970's, which will definitely be faster than a single 7970 in all respects, unless the game uses more than 1GB of graphics memory.
So, two 5970's to two 7970/R9 280X cards is acceptable, but I wouldn't expect an increase in performance in all games, or even most.
I'd expect a decrease across the board with two R9 270X cards.