GregN:
@xanok, I will look out for those, also glad to hear they are strong enough but I'm also worried about reaction speed, I have read numerous reports of ones struggling to find equilibrium and blaming it on motor speed?

Im not expert, but I bought and search a lot of these motors on internet.
As you can see, it's says '270rpm'. a simple math problem.
60s = 270turns
1s = x

x = 4,5 turns per sec

4,5 turns take 1000ms
1 turn take x
x = 222,22ms

in degree:
1 turn = 360º

360º = 222,22ms
1º = x
x = 0,61ms per degree

If you control by time, I think you can use 0,61ms per degree.

Speed can be calculated with:
270rpm / 60 * d * 3,1415
If your wheel has a diameter of 5 cm (d), your speed is 70,68 cm/seg
If your wheel has a diameter of 1 m (d), your speed is 14,13 m/seg

Speed is inversely proportional to torque.
More and more speed, less weight you can carry.

Im not sure about torque equation...
I think if it's says "torque 2KG.CM", it means...
If you have a wheel with 5 cm (diameter), you can use this equation:
torque/(d/2)
or
torque / (r)

2kg / 2,5 = 0,8kg per motor
If you have 2 motors, you can carry 1,6kg.

The problem is, I do not know if this torque is the total limit of the engine, or is the limit to keep the engine specified RPM.