I think it will probably be feaseable it replace a 5-sp with a 6-sp since most of the time gearboxes have the same mounting flange and body mounting points within the same car line. However I see several obstacles. The first would be the cost of obtaining and the labor of replacing the gearbox and shift linkage. In the case where you can find a salvage car and do the labor yourself, it could make sense. The second is the current availability of the gearbox, there are few cars on the road with it installed. Both the Focus ST-170 (Focus SVT in the US) and the MINI Cooper "S" use the same gearbox, with the Ford model having 5% shorter (numerically higher) final ratios. So it will be in short supply for some time to come, I would imagine.
The final hurdle is how you intend to drive your Cooper. If you do mostly 1st-3rd gear driving, the 6-sp. could be detrimental for its taller gearing compared to the 5-sp. If, however, you do mostly high speed motoring, then the taller 5th plus 6th gear would be an advantage.
Jerry |