I would welcome an improved search capability within Mail. I wanted to like Rocketbox, but I found that I could not adequately define search terms to find what I wanted. For example, I was searching for messages related to "parking" sent to a client in "Anytown" (name changed to protect the innocent). Rocketbox returned messages with the word "park," even when I enclosed the word parking in quotes in the search field. I was also unable to limit the search to messages sent to the "anytown" domain, and instead would get all messages that mentioned Anytown.
In comparison, a search in Mail sorted by rank found the messages I wanted. I like the inline preview that Rocketbox offers, but Mail gave me better search results and was not any slower than Rocketbox. In fact, because Mail searches immediately while Rocketbox requires that I press the Return key to initiate the search, one could argue that Mail is faster.
I tried several other searches with comparable results. So for now, I'll stick with Mail.