Hi, I have this set, the motor that came with this set is actually a new "M"- motor, it's more powerful but smaller than XL motor. After building it, if you ask me whether you can mod it with XL motor, I would say it's very, very difficult  simply because, this model is TIGHT! Every part is constructed very precisely, and I believe LEGO has to make a new motor just so that they can make this model! Plus, there's a new servo motor that serves to steer all four wheels at the same time (yes it has actual 4-wheel steering!)

I honestly think this is one of the best technic set LEGO has ever released. The gearing system is mind blowing. Two gearing for each wheel, and 8 gearings for each motor, so total number of gears to move this beast is 32! Also the suspension system is brilliantly done. I would say this model is more like an engineering product than a toy.

This is a highly recommended set. You will have fun playing it as well as building it. Seeing how other technic set is priced almost as high, i think it's justified to get this set  just on the fun playing factor alone. Plus you get all the power functions stuff!

anyone want to build mini tumbler?
here it is http://imgur.com/a/9N2hQ#1