I don't think the T56 is really goign to rob more power from the engine, and being that the gearing is closer ratio it should improve acceleration despite being heavier.
Yes, it will rob some power and the weight difference is about 40 or 50 lbs if I recall.
I believe the Termy T56 has a .62 6th, no it's not really low but it will still save a couple hundred RPM.
No the termi T56 has a 0.63 6th.
Also you will not need to replace the pinion flange and the U-joint, it'll be one or the other.
Are you sure? I was told by somebody that's already done the swap that you needed to do both.