Is the heat for stress relief, or initial polishing?
It is likely to make the surface rough, rather than polishing it. Those diamond saws leave the stone so smooth that it looks polished. If the slabs are to be used for surfaces like walkways or stairs, they are smooth enough to be hazardous to walk on when they get wet. Flaming the surface causes moisture just below the surface of the slab to boil and spall off tiny chunks of the stone. That leaves the surface flat, but with enough texture that it is not a slipping hazard.
I have friends in the granite quarry business. The smoothness of surface that those saws leave is amazing. I'll have to see if I can dig up some photos.