can someone explain to me why the cab is about 2 tenths slower than the coupe.

The coupe is about 4.8 and the cab is about 5.0... why the difference? weight? aerodynamics? both?

The weight is the most likely culprit.

Weight. Don't know the exact numbers but cab weighs about 100kg (220 pounds) more than the coupe because of all the extra strengthening in the chassis to keep it as rigid as the coupe.

Michael.