A couple of things to keep in mind with the BBQ mats - if they are silicone coated, be sure your iron doesn't exceed about 450 degrees F. If they have a fiber weave in them, replace when that fiber breaches the exterior (and don't cut them) - usually that fiber is fiberglass which you don't want to ingest (mostly important for cooking applications, but you still don't want fiberglass in your quilts).
Be sure as OP did to use them on a heat-proof surface like your ironing board or a pressing mat.