You would be supprised at what the 10mph & 1K pounds will do! Stock I noticed a difference between towing accross Ohio at 65 and going up into Michigan towing 75-80, so I'm sure the same holds true for it with a chip.
I dont know what will help you because I only got 11 towing 4K at 70-75mph on mostly level ground with my six gun PDA. That mileage was an average over the whole 850 mile trip.Not that impressive for the money spent.Thats why I ditched it and went with the stealth 2 . I just went 300 miles and got 15.3 towing 3K at 65mph.