I wouldn't be so sure. I've worked with TONS of Emperors (working at the LFS), and the way the water flows through the filter, it is designed to flow behind and through a cartridge. Cramming a sponge in there will cause the water to just flow over top of it IMO.
I don't think that's the case. I know that plenty of people here use foam in Marineland filters. I don't plan to replace the filter carts until they fall apart anyway. They are good mechanical filters and I just rinse them in change water to get the gunk out of them. The carbon is useless to me and since the carbon never releases what it collects, it's just added biomedia. If I need to clean out meds or something, then I'll get new carts.
At any rate, they are both good filters and will keep a tank very clean if properly maintained.