As Uttar explained, there is still more performance left in G80 that will become prevalent with driver releases / optimizations. So, imo, a supposed +5% - +10% R600 lead could turn into a 0% or even -0% after such driver optimizations release by the time R600 is ready to launch in late March.
Granted, there will also be room for R600 to grow, as well. But by the time that took place, I'm assuming it'll be time to start discussing G81, since it is a few month maturity process.
Nontheless, it will be interesting to see how R600's massive bandwith will be utilized. And hearing that the top-end MSRP will be $599, it will definately be more competitve vs. a $549 8800GTX (though final performance will determine that I suppose).
I'm just happy to hear we have a launch date. Now I can atleast have a date to count-down to.
Nelsieus