I despise expanding foam. I personally find it awful, awful stuff to work with. Urgh. I'm talking, though, about the inner plastic trim cover, where the wires are underneath running down the sills. There's a lot of noise comes in through there. You can relatively easily insulate quite a lot of that. Underlay is something certain people swear by, I can't say I've used it. Thicker felt sheets where there's space for them can be extremely effective (especially if you've got some MLV at the base).
Regarding mass vs blocker: you don't *need* the mass unless you're talking bass (and most road noise is traditionally seen as 125Hz, which is close to bass). At the end of the day, butyl will block and add mass at the same time. If you've got space for MLV, great. I don't. You need to decouple MLV somehow, you need high density - which increases thickness again, 4mm for 7,5kg/m2 - and then you really ought to decouple from the interior trim. It adds cost, effort and weight, and yes, I dare say the improvement might be good but only if you do the whole thing.
STP's "Bomb" (which is a very similar product to the multilayer linked) does a decent job of combining mass with butyl. Avoid bitumen unless you like a sticky mess in the car on a hot day.
- Bret
