All very interesting theorys.

I determine servos by the size of the control surface regardless of how fast I intend to fly. The Funana S 90 states a 50 oz as a minimum. This is too low IMO, but it will work. The book states 80oz as optimum, but I just figure, if the minimum required is 50oz, then go with atleast a 100oz. I have HS-5645MG digitals which produce 168oz torque. Just have to be mindful of weight of the servo if it is a 3D plane.

You can never have too much torque. A servo working at it's max will burn out quicker due to the servo having to work harder all the time, instead of working with much less effort, but the strength is there if it is needed.

In my Fat Free Taco (.25 size fun-fly) which has big control surfaces for it's size, I use a HS-85MG (49oz torque), but all that is required is about 25-30oz. I'd always choose to have more, than barely enough.

On a plane that has small surfaces such as my Super Stick 40, I use standard servos, HS-425 (47 oz torque) because the surfaces are small and will not require a lot of power to be moved even at a fast pace of speed.

Determine the servo more by the control surface size and less from the intended use of the plane.
Big Surfaces with speed = Hi servo torque
Big Surfaces with low speed = Semi Hi torque servo
Small surfaces with speed = Medium servo torque
Small surfaces with low speed = Low Servo torque

Big ailerons doing semi fast walls will put a ton of force on the aileron servo. Blenders, parachutes, flatspins, etc... will all require stronger servos on the tail section.