... properly overhauled and tweeked by the master, is capable of certainly under 10 seconds, and maybe less than 5 seconds per day.
I've been wearing a 6105-cased HOT with 5626 "B" movement for almost 2 years now and it's consistently within 5 seconds per day. Really it's about minus 3 seconds total over a week. Exceptionally good! However, getting that last little bit of adjustment ON YOUR WRIST means that you, or a skilled jeweler, have to do the final adjustment as you wear it. And of course, you'll want a good reference clock against which to track it before you make the TINY adjustments on the regulator.
Everybody's different and little things like temperature, or how you set it down at night, make a difference in stability of rate. I do think that the "A" or "C" suffix means that the parts pick was just more selective for that particular movement - not unlike building a "match grade" firearm. It's the same movement, but the good ones had every critical part selected for manufacturing tolerances AND recieved that much more careful attention by skilled hands.
This 5626B is 28.8 and 25 jewels. The other variable in the movements is the type of regulator - base screw, eccentric (like this one-see pic) or the rack and pinion one that can be set from outside the case.
A couple caveats - these watches are not particularly friendly to the day/date QS mechanism. There's a little plastic part that wears out / cold flows after 20+ years and that part is NLA (no longer available). So mine is a non-date. And I generally do not wear it when riding my bicycle.
Just my $ 0.02 / YMMV / disclaimers etc etc.
Bainbridge Island, WA