Ah, well I think I'm improperly assuming your sump is like mine; an acrylic custom piece that needs the foam. I'm betting you're using a stock tank which has a trim that was retrofitted as a sump?
If you think the foam has enough "crushing" capability to level out the tank, I'd place a piece of plywood (sealed, of course) between the tank and the foam. This way the sump would get appropriate support. The (maybe better) solution would be to just use the new piece of plywood and shim that until it's level.
I will reiterate something that I've felt true for a long time; a tank doesnt necessary have to sit on a perfectly level surface, but should be on a flat surface. Of course if there's a 5-10 degree slant, then you start to have weight distribution problems on seams of the tank, but a flat surface that has a ~1 degree or so lean to it should be of little concern. If you could get the new piece of plywood relatively level in the stand, I'd be content to go from there. Especially on a smaller volume sump.
