At our company, we've found that a cohesive design throughout a funnel will always* drive better results. If you think about google as a step in your funnel, then you'll be improving conversion through that same principle.

Also, you've guaranteed familiarity by using Google to buy users. It might be interested to buy users via Bing or Yahoo or something, to see if that same design wins.

But, that's purely gravy. You've probably hit a pretty good balance of testing and results with your experiment already.

* "always" -- not really, but usually.