The original foam bits are so flimsy that they don't serve any useful purpose in my experience. The foam strips I've added are only about 10mm thick, and compress quite easily down to ~1mm.
The chances of generically shaped helmets fitting anyones head with no to minimal clearance across a large area are, IMHO, not great. No foam helmet I'ved used- and that's quite a few over the past 40 years has fitted my head very well. Maybe I have a weird shaped head  They rely on the rather flimsy plastic adjuster strap around the back of the head to obtain fit plus a few other small contact points, no doubt varying dependent on the shape of your head. The best fitting one I've used, but probabaly least protective, was the old leather one in the late 70's, I think filled with horse hair, that had 5 or 6 longitudinal ~20mm thick sections, plus a couple of bits of foam that went behind the ears.
 it certainly did change shape a bit after my 5 skull fractures with a standard helmet!