Chris Venhaus' rule of thumb is - use shielded power cords on digital equipment; use unshielded power cords on analog equipment. I think he bases this on extensive listening tests with his many designs. That seems consistent with your the above theories, though I suspect it's a YMMV situation depending upon system setup and how bad/good the power designs.
I swore by shielded ICs, but I've been slowly replacing them with unshielded ICs, just based on doing A/B listening tests, sometimes blind by asking a friend (or wife in a particularly patient mood) to swap them for me and not tell me which is which.
Side note on grammar - I'm somewhat over-educated, but I often do need to think twice about "effect" vs. "affect". Thanks jeffreybehr, that's the simplest most elegant explanation of the difference I've ever heard, at least for when used as a verb. I'd post that on my office cork board, but for the embarrassment, perhaps just a note in my wallet. I appreciate being corrected. My PPs are the constant misuse of "literally" and people using "may" when they mean "might" because "may" sounds fancier even though it's wrong, Just the other day I heard on the news about how so and so "may" die. Yeah, well we all "may" die. News flash, someone is not immortal.