From my experience I typically go with:
150oz/in at .14 for a buggy
200-250oz/in at .16 for monster truck/heavy truggy
300oz/in at .18 for rock crawlers/VERY large MT's
All with a metal geartrain.

Just my guideline when buyin a servo. Of course my 1/5 scale has it's own requirements.
And SUPER ULTRA HIGH SPEED 6S servos are just overkill IMHO for anything less than a pro racer that gets paid to win,and even then the servo isn't always better.