I'm getting about 19-21mpg pretty consistently. I have heard of people getting up to 23mpg, so I think part of my problem is California smog equipment and a crappy ethanol fuel mixture.
I test drove a 2.0L and 2.5L when I bought mine, and I noticed a quit a difference with the V6. It brings the car to life. Don't forget, if you get a 2002+ model then Suzuki has already installed a larger exhaust system to give you +10hp

As far as towing limits, I'm not completely sure. It is rated at 1500lbs, but I have always thought that is very low. The engine could definately handle more than that. I'm not sure if the suspension, or brakes would need to be upgraded, or if the engineers just wanted to be conservative on the tow rating. I believe in Australia the GV is allowed to tow up to 3000 lbs.