I always take "Novak's brushless gearing advice" as a starting point.
http://www.teamnovak.com/tech_info/b...r_gearing.html
for a 8.5T on a 4wd buggy says a FDR of 9 so you will need a 20-21 pinion.
Even that the best tuning tool it's a thermomether. Do yourself a favor and buy one. It's cheap (10-20$) but a lot useful.
I use a 120 xerun v2.0 on my durga with SP v3 6.5 T and the firmware of the ESC had a major impact on temperature. Try *205g or *205MOD. firmware ( you can upgrade firmware with the lcd program box and a PC).
If the temperature problem persist, check slipper (maybe too thigt), and/or decrease timing.
Hope this helps