I just got outpulled by a 6. 0 towing in Montana this weekend. His trailer might have been a little smaller than my 5'er (and ALL of my tanks were full), but I didnt think it was enough to make as much of a difference as it turned out to be. I had to back off a little as the EGTs were hitting the 1250 mark. He blew some smoke right off the bat, so I presume he wasnt stock either.....
So I'd hold off on bashing the other brands (although it is fun!) until you've actually run against someone who is intentionally trying to pull you.....