Towing with an E70 4.8 vs E53 4.4?

[TowX]

Wondering if anyone has experienced the towing difference between an E53 4.4 and the E70 4.8. My E53 pulled my old open deck race car trailer with ease. This past race season I decided to get an enclosed 20' trailer and the E53 doesn't have enough torque to pull it very well. Granted the new trailer with car and tools weighs close to 6,000 lbs. Leaving the Step trans in 5th, it barely pulls hills without requiring a downshift to 4th, at 70 on the interstate. Not sure how other E53 owners got away with pulling enclosed trailers as some had said they were.

So I was wondering if anyone here had ever gone from a 4.4 to a 4.8 and how much more pulling torque was available. I'd really like to stick with an X5 but may have to look at some 2 and 3 yr old Touareg turbodiesels, or even a Yukon Denali.