I haven’t but am interested so read some on the prusa forum

Tungsten carbide: Mohs’ hardness=9.09, thermal conductivity=110 W/m/K, heat capacity (volumetric)=2.9 MJ/kg/m^3

Crystalline alumina (ruby, sapphire): Mohs’ hardness=9, thermal conductivity=30 W/m/K, heat capacity=3 MJ/kg/m^3

Looks like they’re very close to equal in hardness (note there’s several ways of measuring this, Mohs’ seems reasonable for abrasion resistance), a given sized chunk of each will have basically the same heat capacity, and the thermal conductivity of WC is far better.

It’s the thermal conductivity that helps TC be better than ruby. The problem with printing using the ruby is getting the ruby small enough it doesn’t reduce heat flow to a point it won’t support reasonable material flow.

So because the thermal conductivity of Ruby is low you want to reduce its thickness which can be hard due to cracking.

Also it’s my understanding that the upper part of the nozzle is lower pressure and primarily for melting the filament, then the pressure increases as the inner diameter is reduced.