RTV as muffler patch?

OK guys, ultra copper RTV (best stuff?) is good to 700F (shows bead on header flange!) and the black stuff is rated for 400F-500 intermittant, No good. I took temp of pipe into muffler and it was 470 with just 10 minutes of stop and go errand running on side streets, cat was over 500. Muffler case was 270. Im out of copper so Im going to get the muffler patch in a tube, good for 1000-1500F intermittant and cheap at $3.50. The Muff is $47 but its all welded so I cant just unbolt a flange. The muff is in good shape (weighed a ton when I took it out at last repair, full of rust??) except for the small leak at the pipe head. shop wanted $90 so I figure Ill just deposit $85 into the Mopar fund :-)