More like 80-85 hp in reality.....
But regardless, you are correct. Most of the modern 5 and 6 speed trannys are designed with fuel economy in mind. So we're talking about double and triple overdrives, which an Opel with a 3.44 final drive CAN'T pull. And I'm curious, how was the transverse Mini cooper tranny (front drive) going to be installed in a rear-drive car? Anyway, the only logical application for a 6-speed in an Opel is either:
A. Accompanying an engine swap, or
B. Using a true close-ratio racing 6-speed. In this case, the 6th gear ratio is usually 1:1, and all the prior gears are merely closer together for better acceleration. Top speed would be a matter of HP, final drive gear, and aerodynamics. Fuel economy would be no better than a 4-speed's.
Imagine fitting a ZF tranny from a 'Vette? With a .50 overdrive in 6th gear, you'd get a 1.72 final drive in your Opel! The only Opel I know of that can pull this is a Lotus Opel Omega...with a 360 hp twin-turbo straight six.
Bob