[Marcello]

Game weight and carry capacity is more about preventing an helicopter which can carry a BMD from being able to lift a Maus as well, rather than an accurate reflection of actual weight/carry capacity. Tentatively one ton equals one point IIRC, but this is more a starting point for further fine tuning than an hard value.Also crew is added to the weight for carry capacity IIRC, so keep that in mind.

"How do you come up with values for weapons if they never got off the drawing board?"

Without a penetration table I would not dare to touch the issue with a barge pole, but if you have worked out the AP penetration the rest is relatively easy. HEAT has already been dealt with. For Sabot do you mean APCR or proper APDS?
What's the timeline?