Yeah,
that's a curious point to make on the performance docs. I'm not sure I've ever seen a sp3 specific performance guide, just the 6.x version. Frankly, those numbers aren't as relevant as they should be, since today's servers aren't even using the same cpu/chipsets that were tested in the lab. Toss in virtualization, multi-cores, etc...and you can see that the perf. #'s aren't exactly apples to apples.
I'm not saying that the doc is bunk, but rather that your mileage will vary. Standard deviations to or from their norms in the guide are to be expected at the very least.
And yeah, with that said, still upgrade. Stability is key, and sp3 delivers a lot more then sp1 or 2 had.
micah