All indications are that the 20-30% average is for the 512SP part. It wouldn't make any sense for the 448SP part, as it would mean a 50% increase for the 512SP part. I very much doubt you will see the kind of improvement from Fermi to 5870 on average that the 5870 sees over the 4890 (+- 10%). In their benchmarks nVidia never specified that it was the GTX 360 that was giving that kind of improvement in Far Cry & Uniengine. Let's go ahead and assume that if they could talk those benches up more, they would have.
And it isn't very impressive considering that Fermi has 50% more transistors, although obviously if the price is correct it won't matter for anyone beside nVidia and the server-class market.
edit: It isn't right to take Razor1's comments on B3D and use them as fact, just like it isn't a great idea to take ChrisRay's comments on Fermi too seriously. The latter is the same person that said all of our questions would be answered come the Sunday reveal, and that Fermi is so great, that the people who doubt nVidia's focus on the consumer games market will be silenced. Then the reveal happens and he backpedals immediately. It's the same reason you shouldn't read Charlie's comments as absolute fact. The difference is guys like Chris Ray have some more credibility, but are actually significantly more biased towards one company. Not to pick on him, but he is close enough to nVidia that I wouldn't go by his notions. I'm not sure where Razor gets his information, but it doesn't seem to be right in this case.
Then you are in for one hell of a shock come march then as the part that has been shown to date is in fact the 448SP part. The 512SP parts are, it turns out, going to be coming from B1 steppings as the A3s haven't made them any more viable than A2s, EI 448SP working parts are what they are getting from TSMC in quanity.