The heaviest I have to put on the head is a d80 with 300 f4 prime, it holds this very stable but then I think all the top offerings will easily do this. The main critisism of the cheaper ballhead was actually the plate interface, the one that came with it was made of cheese an wouldn't hold anything stable, also the ball lock would slightly move the camera position...
The latest Z1 that you'd get has been revised, the pan lock is now a small flip leaver making it easier to operate when on larger tripod bases. It's movement is like from about 10 o'clock open to 2 o'clock closed. It doesn't completely lock out but does enough that it would take a lot of force to rotate (when mounting the head on your tripod you want to grip the very base).
The main lock knob is nice and smooth, it has a good range of adjustability (is that a word?), you'd be looking at a usable range from about 4.5 / 5 and on for a D3 I'd imagine. The minimum lock adjustment is slightly different to the RSS from what I can tell. It's actually a recessed thumb screw in the main lock knob, basically set to the required tension on the main knob and then thumb in the min lock knob, the main knob now doesn't turn below this position. This is good as your only working on the main lock knobs range of hold (and it's nice an progressive). By the higher numbers like 9 it is locked very very tight.
The other nice feature is the elliptical ball, as the camera moves more off-center the lock becomes tighter, can be handy when you have it at a medium setting, you can take pics then move and take more pics often without needing to touch the lock again including up and down motions. Although this extra amount of lock is probably only suitable for shorter lighter lenses than a 200-400
I guess the only issue I found was it took a couple of days use for the ball and socket to properly seat in. Also when panning there is a slight tight spot but you'd never really tell under normal use.
Fire away with any other questions.
You can see the recessed min lock adjustment on the main knob here. I can't find a pic of the revised pan lock, I'll take a pic of mine tonight.