I have a 2006 Pro upgraded with 2 quad-core 5355s, and using the SL beta (build 411) has been a real pleasure. Apps load faster, and the whole system feels snappy and responsive.
The only real benchmarks I've run is for Safari, and there is a distinct improvement in 64-bit SunSpider benchmark scores over Leopard on the same hardware (430 ms vs. 557 ms). CPU utilization of non-SL-optimized apps (Skype 2.8, for instance) is lower in SL (6-7% vs. 12% for video).
It's absolutely true, though - whatever application you're using needs to be 64-bit and recompiled for SL (Grand Central/OpenCL aware) to really see a huge improvement. But if you are using apps that need to wring the very last drop of performance out of your Mac, then I would pretty much guarantee that the developers will release a new version when SL is out.
Unfortunately, as in the case of Adobe CS5, they may charge a fortune for it.
Hope that helps, JP