The Steel Exhaust Isolating Mat is legal for all Rotax senior so do not worry about that. I think this has been the case since the 2012 season. You said you race in the TAG class so you probably see the Leopard powered karts get warmed up rather aggressively on the stands and think you better do the same! In PA we have similar weather or even colder sometimes. All we do is add two or three strips of tape on the radiator, Start the kart about 3 or so minutes before rolling off the grid and we get it to temp with in a lap or so. This is sometimes in weather cold enough that we add latex gloves inside the regular gloves to keep feel in the fingers! I am assuming that you have the new radiator with the flap to modulate airflow. You are at MMX yes? I love that track! Talk to Eric Jones, he will help you plenty and keep you Rotax legal.