I filled up my driver's side b-pillar where I could. It marginally helped but I feel I got better results putting a layer of CCF and MLV in between the plastic panel and the pillar itself.
As noted, it takes a crapton of foam to fill up a decent size area. It's actually more cost effective to use CCF and MLV. Unless you find some foam that REALLY expands a bunch, which I haven't found yet (but would like to). Only downside is you have to be extremely careful to not use too much or else you'll end up warping panels.