Can anyone give us a quick rundown on the tortional strength of a tube in relation to it's OD?

What I really need to know is how much i'm gaining on torsional strength by going hollow as oposed to solid, and where do you draw the line.

Weight and strength in torsion is critical in this aplication.
Thanks
Pieter

You're not gaining on torsional strength by going hollow, but losing weigth and a little strength.

The formula to compare is, for centerpoint in the middle:

Solid:
Ip = Pi / 32 * D^4

Hollow:
Ip = Pi / 32 * (D^4 - d^4)

You will see that all the strength is concentrated on the outside, so beware of "tin foiling" your design.

Torsional stiffness increases with the fourth power of diameter, so for the same weight of material, a tube of double diameter will have roughly 16 time the torsional stiffness.

This works both ways, when looking at propellor shafts, a little bit too thin can be a lot too weak.

If your design is limited to a set OD, and weight and torsional stiffness being critical, then moving to a higher strength alloy, and a thinner walled tube will enable you to improve both parameters.

Is there another way of determining that turning point, or should i continue on the "that looks about right" path?

You've got the formula, the continuation of the "that looks about right path" is something you have to discuss with your insurance company.