TweakersGuide has posted an article about the second beta of DirectX 9. The article shows comparisons to the first beta as well as to DirectX 8.1. Several benchmarks are included.
"The second beta performs like the first one: not better than DirectX 8.1.
Thus, it's not really necessary to install this beta unless you are a developer and might want to check out DirectX 9 as the leaked betas are SDKs (Software Development Kits)."