R30 = heavy
Only reason you'd need a heavy R30 box would be because you're putting a bigger, heavier (more powerful) engine in. Thus the weight of the gearbox is not all that important, because you've upset the balance of a car that pretty much depends on its light weight to do things well. Of course it will accelerate, but it may not handle.

On the other hand, if you're lying under the car trying to lift it, the weight of a gearbox is a CRITICAL factor