Thanks for the reply!

I actually ended up going with the 22T 4.0, it's been interesting...

It's running a stock spec 17.5T motor, so definitely no speed demon (looking to swap in a quicker motor), but surprisingly the handling has been a delight, even on grass. The wide stance means it's almost impossible to traction roll, and I've found the slidey-ness of the RWD setup to be manageable. More than anything the precise steering response and predicable handling have been what i've been most impressed with.

On the other hand I also have an old WLToys 124017 that I've been bashing with - 4WD brushless. Its certainly a lot quicker in a straight line, but really more of a point and shoot type experience when bashing on grass seeing as it has very limited ability to hold a corner at any speed.

From a dog perspective, after some extended sessions she tires pretty quick, so top speed is only needed briefly and then it's more a case of twists and turns to keep her interested. In saying that, the 22T in it's current form is too slow for when she gets a burst of speed. Im planning on trying some Pro-Line Trenchers (so I can use the original 2.2" rims), and/or some JConcepts Magma premounts (Rustler size), which I think should give a lot more bite in the grass - its currently on Prism type tires in the rear.

Will post updates as they come!