It appears that the configuration of gearboxes regarding the ability to use of H-pattern and clutch is inconsistent, or maybe it's that there is some internal GT6 logic that I can't quite grasp.
One case to illustrate this is the McLaren F1 '94 and the McLaren F1 GTR '95, both premium cars. In the F1 94 you can use the clutch and H-pattern no matter what gearbox is installed (stock, fixed 6-speed or customizable 6-speed), while the F1 GTR '95 only has the customizable 6-speed and it is sequential: no working clutch and no H-pattern shifter. However, from what I've read about these cars, the F1 GTR '95 was a racing modification of the F1 '94 and the only changes made were the addition of aerodynamic parts, weight reduction by stripping the interior and supressing things like the stereo music system, and an air intake restriction to limit power to 600 HP in order to comply with racing class regulations. So apparently, there should no major difference in the adjustable gearbox of both cars, however GT6 allows using the clutch and H-pattern in the F1 '94, but not in the F1 GTR '95.
I have just added a TH8 shifter to my T500 setup and discovering these issues with clutch operation and use of the H-pattern shifter is annoying me a bit.
Cheers,
S.