I think the long and short if it is... while people insist on using the original case design for more than 4 gears, then it's going to be a compromise on the strength of the unit, as the case is not designed to carry load bearing gears on the speedo side of the mainshaft...
At least the KAD 5 speed retains the gearset in the area the gears are designed to be...
The solution is design a new case, with a new gearset, and I'd be surprised if anyone is going to invest that kind of money into a legacy unit.