In 1/10th scale buggy I think 90oz.-in. is fine, just get a good one with metal gears for the hard hits and bearings for smoothness. 1/10th 4wd buggy I like a little more torque (like 120-140 oz.-in.) because you have the added friction from the front u-joints and the added inertia from the front wheels being under power. Also 4wd puts more weight on the front axle so tire friction becomes an issue.
To me speed is more a response thing. I find faster servos make me feel more "connected" to the car because there is less lag from my input at the radio to actual steering response of the car.
1/8th scale has bigger tires, bigger u-joints, and bigger hubs which equal more resistance to movement. Bigger, heavier wheels spinning just as fast if not faster equals major jump in inertia and thus higher torque to efficiently move the wheel. Ever spun a bicycle wheel while holding it in your hands then tried to turn it on the vertical axis? Not as easy as it sounds...