Anybody ever try Mr. Clean Magic Eraser? You may laugh, but some of the dumbest things I've heard actually work .... never tried it myself however.
I drive tri-axle dump for an asphalt co. and we use a chemical I know would take it RIGHT off without damage made by Bio-Solve or something. We fill our spray bottles up with it for state jobs when you're not allowed to use (diesel) fuel which also works. The stuff takes tack/asphalt right off my wheels which get caked bad from running it the stuff constantly when we pave. I spray it on all my wheels and just run with it soaking all day, looks like crap til I clean it all off, but they look like new when done.
Same goes for your exhaust no matter what you use ... soak it down with whatever your poison is first, fire it up JUST long enough for it to start "steaming" then have at it. The heat will help loosen/soften things a bit being careful not to get it hot enough to burn yourself.
No need for 0000 steel wool or anything metal to try scraping it off. I've heard of the coin trick from old timers, but I'm too anal about my stuff to even think about trying that one.
I have also used Nevr-Dull & WD-40 with good results too. Pick whatever poison you have on hand or run out and buy something new and use whatever works best for you.