Shock length IS a problem.
The standard length produces around 17 degrees of U joint deflection.
There is a limit on the U joint deflection where the U joint will make internal contact between one side and the other.
In addition to that there is also a velocity (speed) variation between input and ouput htat is 0 for 0 degrees deflection, but increases with incresed deflection angle.
The standard G/S is at about the limit of the U joint deflection. For that reason I'd not go beyond it.
Regards Frank Warner
motorcycles BMW R80 G/S 1981, BMW K11LT 1993, BMW K75 G/S