'11 GC Overland, Hemi, HD Tow Package
I tow my '00 XJ. Fully dressed I would estimate my load at or less than 6500 lbs. The trailer has electric brakes on both axles (brake controller in cab), and I use a weight distribution hitch. I'm at about 2,000 miles this year with towing.
I do the majority of my towing at 65 MPH so I can minimize fuel consumption. However, I have towed faster than that, and have always felt secure. The electric trailer brakes are awesome, and I don't feel that the brakes on the Grand are stressed because of this. The QL and Auto-Leveling suspension do a great job at keeping everything balanced. The anti-sway control that the Grand is equipped with also makes for easy towing.
The trans cooler is sufficient, but in very mountainous driving it will warm up a bit, so a bigger cooler is in my future (probably Summer '12). I use Tow Mode regularly when hauling and have found that doing so does keep everything cooler, and keeps the trans from finding gears.
I have found the Hemi to definitely have enough power, and can hold 65-70 mph on steep climbs without the motor screaming.
If I haul conservatively I can do 12 mpg, and if not I'll average 11 mpg.