I have Model Ms, aluminum Filcos (heavier than the Model M), heavy Black Widows, older Das, lightweight Vortex Race, and a Magicforce.
I used to want heavy because i thought it equaled stability. which the Vortex cased Filco provided, but after getting the Magicforce, I found that's not actually true. It's not a matter of weight, it's a matter of stiffness and quality of the feet. Get a good stiff case with some good sticky feet under it to keep it planet, and you won't need a really heavy case. Granted, in most cases a good, stiff case = weight, but it doesn't have to be. Take a poorly made aluminum 60% case that doesn't support the PCB and it will do little to make it feel any better than the plastic case.
Point is, don't be concerned so much with the weight, but more about how stifff the overall keyboard is. The Race had a plastic case, no plate, the Magicforce has a plastic case, with an aluminum plate, the Magicforce feels stiff as heck while the Race feels like a wet noodle. On the other hand, the stock Filco (TKL), Das (TK) and Black Widow (TK)were stiff at the keys, but the cases flexed, the Filco and Das much less so due to better case and feet construction.
I would love to see what a well made carbon fiber case, with aluminum Plate would feel like, should be fantastic.