Yeah, 5870 is not as based on 4870 as 580 is on 480, I'll give you that. But the fact remains that it was still VLIW 5; and mostly the same with 4870 except the tessellator unit and the added SP's. The grouping of stream processors, and the processors' capabilities (and general dependence on the performance of the scheduler, I assume) hadn't changed.

5870 by itself is the fourth iteration of the same arch. Obviously it's not going to be ENTIRELY the same. But before saying "it's going to be awesome with drivers" and possibly misleading people, think of the last time AMD introduced a new arch; X2900XT. When it was released, everyone was disappointed at the performance, and a lot of people encouraged people to buy the card with "it's going to get better with drivers", whereas it didn't. It got better with iterations.

X2900XT was a new arch, as was GTX 480. Those products both sucked (2900xt sucking more, I might add). The next versions, HD3870 and GTX 580, were 10x better received by everyone, and were the products they caught up with the competition. There is a similarity.