If they really released a "premium version" with a higher resolution screen but with the same CPU and graphics, wouldn't it affect performance and battery life?
Yes and yes. Driving more pixels is going to be harder on the GPU. This doesn't really make a whole lot of sense in my opinion. All those new iPad 2 games that come with higher detail graphics are not going to be able to upgrade to a retina display without bringing the detail back down again.
In my opinion, a retina display should only come with a GPU upgrade.