I used to cut the foam (V28 is the grey stuff you're in about) for DFS sofas and we used big (52" of exposed blade) band knives with moving tables. We had a CNC machine for cutting out shapes and that again was a band knife (Fecken Kirfel was the make, or spelt something like that) but with 6 steppers to drive the blade and twist it. If you try to use a simple blade then as soon as it gets blunt (which it will quickly) it will tear the foam. You'll be better with a reciprocating blade or a band knife and moving/swivelling table.