Lane Keister over at Green Baggins has posted a very good argument by Owen defending the Protestant belief (against RC) about the way in which the canon is discerned. Naturally, we could extend this a fortiori defense to encompass the perspicuity of Scripture and other doctrines relates to sola scriptura. I think, however, one can push the natural/supernatural analogy further and use it to internally critique what generally passes for RC epistemology. Matthew Schultz at BeggarsAll touches on this point in final paragraph of his most recent post on Whitaker's "Dispuations."
RCs predicate the justification of the canon on an allegedly infallible church. Allowing this, by what means do RCs justify their knowledge of an infallible church? If the Magisterium requires no external justification, then we can, like Matthew, ask what distinguishes Scripture from the Magisterium such that the former cannot be afforded the same presuppositional privilege as the latter. [We might also ask what differentiates Scripture from, say, God's command to Abraham to sacrifice his son of promise; Abraham did not need to think about whether or not some omnipotent demon was attempting to trick him. Why? Because God's word is self-authenticating, self-verifying.]
However, I can't recall the last time I asked a RC by what means they claim to know the Magisterium is infallible and received a response which didn't in some way appeal to history. Disregarding the fact many of these responses commit the appeal to antiquity fallacy (not to mention that they are only probabilistic arguments), these are, at the very least, responses which implicitly appeal to natural revelation to justify a belief in an infallible authority. We could modify Owen's argument, then, to read: "if Protestant presuppositional epistemology, grounded on supernatural revelation, is unsound, how much more so is RC epistemology, grounded on natural revelation."