The big problem with using stainless steel wire for foam cutting is that it expands quite a lot when heated. That makes it difficult to adjust and maintain the wire tension during cutting. During a cut, the foam material tends to cool the hot wire, making it shorter (tighter). Increasing the Variac voltage (and current) restores the wire temperature, but then the wire sags. Tjhe result is an uneven cut. Foam cutter rigs having some kind of spring to maintain tension helps a little, but steel wire is still difficult to control.
The recommended wire is called "Nichrome" and is available through some of the big mail-order hardware (McMaster-Carr?) stores.
I was in the kit business for a while, and I experimented with foam wire cuters quite a lot.
Floyd in OR