So, there's really not much correlation w/ an IM rating and a modulus rating?
Yep. It depends on who's saying what the IM rating it is.
Do either relate to the sensitivity of a rod?
Yep, the high modulus rod will be more sensitive and lighter but the trade off is that it's brittle. IM ratings help negate that trade off. The rating is the quality of graphite they used, but it's not a standard so you just have to compare them.