trackstar 3.5T
 

I've seen HK are doing the track star 3.5T sensored brushless for £12.99 right now. I have their 9.5 and I can't fault it.

This is for a toy, and I like the idea its insane. If I dialed it down a bit (maybe use turbo settings on ESC and throttle curve on controller) do you think it would be usable in a short course truck on a wide open grass or dirt space?

I just find the 9.5T a bit yawny on a big area. I want to laugh every time it flies past at Mach 10

You want something like a 550 7.5t short course motor
The motor you list is too quick its for modified touring cars or 12th scale
The fastest off road motor people use in 4wd buggys are 5.0t