Didn't someone say that 4D shaders run at something like 90-98% the equivalent in 5D shaders? So 1920 x 4D would be equivalent to nearly 2400 SP's.

Of course, the issue is how much of a bottleneck Cypress had, and we saw that 2 x Juniper often scaled better than 1 x 5870 despite being the equivalent in unit count, so it'll be interesting to see if Cayman uses its GPU much more effectively