I'd go out on a limb to make a case that a 3t Obito would only be as powerful as another fodder Uchiha. Because you're right, we only know Obito for his MS/Rinnegan capabilities. And if you think about it, if it weren't for those he wouldn't have gone far at all.
We only knew about Obito's capabilities as a shinobi before he was crushed by the boulder; back then he was naive, clumsy, and frankly a bit of a wimp. His physical and mental capabilities were sub-par. He was the Uchiha's pre-time skip Naruto (just without the determination and never give up spirit)...
AFTER the boulder incident, Obito was given enhancements up the wazoo... his physical capabilities were upgraded due to Madara/Zetsu. His intelligence grew as a result of Madara's influence and knowledge that was shared through Zetsu. He hid in the shadows. Yes, with the MS Obito is powerful, but without it, a base Obito isn't anything special really.
Another way to test this theory is to rephrase the question: Instead of "how powerful is Obito without Kamui", it should be "how powerful would a fodder [Uchiha] be with Kamui". Because if you can assume that a fodder would have been equally as threatening with Kamui then that really drives my point home.
