I've not yet used a six speed box to tow, but my impression is that it would give a good benifit when on the motorway and you are upto 60mph. The added choice of gear ratio's should make the hills a little smoother too.
I personally don't use my cruise control as it tends to work the engine harder to keep the momentum than I do with sympathy to the engine in mind. You see, I tow at max tow capacity which is hard enough on the engine anyway, beside's that, I'm a petrol head and like to play with the motor. (or is that contorl freak? )
You can expect fuel consumption to drop by up to 10 mpg, but with sympathetic driving you might reduce that to 6mpg. That doesn't mean driving slow but being more perceptive at junctions and avoiding harsh acceleration etc. Waiting for that slight down hill bit of road 500 yards infront can save 3-4 mpg as does going up a hill at mid rev range in the next gear down compared to labouring with the foot to the floor in a higher gear. It's all an experiment and I'm sure you'll have fun playing with the fuel consumption computer and quite suprised at the results.
Steve.