Yes, there is definitely something crazy accurate about these older movements. My '42 is 1 of the most accurate watches I have. Runs well within COSC parameters.
Definitely, if you have a NOS Twinlock, that is the 1 to go with. Although I would never swim/bath with my Subs, I have never had any humidity issues with them. Remember, the Twinlock was intended to seal out moisture down to 660ft! So, as long as it is NOS & fitted correctly, humidity leakage (via the crown) should never be an issue.
At least after a full overhaul, I have noticed the same thing. After a full manual wind (40 turns) months ago, the mainsprings in my GMT & Subs are near full charge every time I check them (& keep excellent time).