Nope. Google uses a ton of WebKit code, submits a lot of code back into WebKit proper, but they're doing their own stuff too. IIRC they wanted to have GPU acceleration for 3D CSS which apparently WebKit proper doesn't offer at this point or Google thinks they can do it better. You can't safely assume all WebKit based browsers are going to be equal.
I've got a radar open with Apple about Safari's performance (that I know WebKit people have at the very least seen) that essentially boils down to this reason.
Granted - Apple's definition of open is "you should only use Apple products".