Floating gears in a synchronized manual is just abusing the synchronizers as a clutch, if you get it right it's only a few rpm but why risk it? There is literally no advantage.
Race car gearboxes like crashboxes and unsynchronized sequential gearboxes have stronger gear dogs with more backlash (bigger gaps between the dogs) to allow them to be shifted without synchronization.
Double clutching is pointless. Just putting more wear on the throwout bearing.
I have been doing all these horribly, scandalously abusive things all my life and have yet to replace a manual gearbox or have a throwout bearing fail prematurely (or at all).
Race car gearboxes like crashboxes and unsynchronized sequential gearboxes have stronger gear dogs with more backlash (bigger gaps between the dogs) to allow them to be shifted without synchronization.
Double clutching is pointless. Just putting more wear on the throwout bearing.