I think you're right about the focus on small government, but the rationale for that focus can vary widely depending on the person - the small government mantra is simply the rallying point.
Also, I think it's entirely possible for the tea party to shift to a 'reform' movement, but given the ease of affiliation and the wide variety of incompatible ideas, I think the result is going to be the same - anti government.
edit:
Consider the 'Taliban'. Not everyone affiliated with the Taliban is pushing to implement Sharia, and moreover, the way things seem to be going, if you're fighting the Afghan government, you're branded as Taliban. End result - Afghan government gets shot at instead of changed in a way that will satisfy its critics. I think the tea party is in a similar situation, without the IERAB and beheadings, of course