We have pulled our Farmall H up to 9000 pounds and won against 666's, 4020's, etc. in a 3mph pace class. Have also had our M up to 14,000 and won against prety much anything you can think of. You can't do it everywhere all of the time, but on the right track with the right sled on the right night with the right tires is a sweet thing!
The track we used to pull the 14 in has really toughened up the past few years, so we've geen stopping at 12,000 and sometimes puking it in that class... Never know if you don't hook and try!