OK ... I'll show my age with an old (like me) trick. Try holding a dollar bill so that it hangs down covering about the top half of your tailpipe. If you burned an exhaust valve you will (most likely) have a negative pressure wave traveling up the pipe and it will try to suck the dollar bill back into the pipe. This (along with the crappy idle) is grounds for pulling the plugs and doing a compression check.
Since the 'tour doesn't have a true dual system it may be hard to do a left/right comparison to narrow down the defective bank (front/rear)...
When you compression test the values should be comperable from cylinder to cylinderplus or minus a few percent... so if normal is 160 pounds and you get 162, 158, 155, 160, 140, 158 ... well you know where the problem is... By the way .. if you do have one low cylinder try squirting a little light oil in through the spark plug hole and rerun the test ... if the compression comes back up is more likely rings than a valve.
My guess is that you did not burn a valve from open headers ... that would be pretty extreme ... but if you bounced off the rev limiter one to many times ... you could have bent one. Good Luck
