I don't think it will, but I think it certainly could happen. I'd even say it would be likely for the iP7. Wouldn't require developers to do anything either.
If they boost it to 1242 x 2208, then it will no longer have to downsample everything to the 1080p display. Performance may actually increase, though that sounds slightly counter intuitive. I'm not sure what the effect on battery life would be, but I don't imagine it would be massive; 1080p to 2K isn't a massive jump.
Maybe I'm missing something, but I think it's plausible.