I don't think it will turn out well. The GTX560TI is already rather close to the GTX570. (the new 560TI features a core similar to the 570, but with a few more bits turned off.) This means depending on the clock speeds of the cards in question it should be between the current GTX560TI and GTX570 in performance.
But because its based on the 570 core its power draw will be closer to the 570 then the 560TI. It's performance will be a bit higher then the stock TI, but its performance per watt will/should be lower. OC'd current TIs should have no problem keeping up with it. Nvidia is probably only doing this because they want in on the holiday season and have no new cards for us, or they have a bunch of chips that can't pass as GTX570s and they want to sell them as something else.