Hi
Think the instructions say no lower than a 8.5T in off-road.
I'm running one of these with a 7.5T in a 2WD Losi 22 without turbo or boost and have had no problems. Running on high grip astro with longish straight. Neither speedo or motor getting particularly hot - motor around 55-60 degrees.
I would have thought you
might get way with a 6.5T in 2WD but think you might be pushing it with a 4WD. I'd run it with the 8.5T and see how hot the speedo gets first.
Be interested to hear how you get on if you do try the 6.5T though as I have one sat in my pit box
Cheers
Rob