What you need to consider is this.
If the GK104 card IS $299, and IS as fast (or marginally faster) than the GTX580, which will put it within spitting distance of the 7970, how will AMD reply?
As far as we know the 7970 is their top-end single GPU offering, yet GK104 will take the mid-high spot, it's very much a mainstream high performance card, nowhere close to nVidia's top offering, although whether we'll see that card before June is questionable right now.
The 6970 was supposed to compete with the GTX580, but it didn't really manage, yet it was AMD's top-end card. GK104 is not meant to compete with 7970, but 7950/7870, yet it may be close to 7970. That would be a big blow for AMD, especially if the price really is around $299.