A ~3mm stainless backplate typically weighs 5-6lb. 6lb = 3mm. Is that what you meant or are you thinking those are two different things?I do like the idea of the 6lb plate, but that might stick with a 3mm for now, just for the cost.
The only heavier backplates I've seen are the 6mm subgravity stainless plates, weighing in at around 10 pounds.