If you replace the A-S clamp with an RRS clamp be careful which model you buy. I( think you will want the BS-AS II according to this page on RRS's site. It's not easy to find; you may have missed it.
I would also ask why you want to replace the A-S clamp.
The Z1 is a good head. You've noted that it is substantially heavier than the Q10 and the Q20. The Z1 is more comparable to the Q20 in terms of performance.
Compared to the M10, in exchange for the weight and potential issues with the clamp, you get a head that does hold extreme angles better because of the elliptical ball. SO there are various feature trade-offs here. I shoot Markins heads and am very happy with them.
I've only recently had a chance to handle a Z1 in the field. From what I Could see in the 5 minutes I spent with it, it is a very nice head. I spent my 5 minutes studying the action of the elliptical ball. I like it better than the A-S B1 I tried a number of years ago.
As far as weight on a Series 2, I think lighter is better, which favors the Q10 (a good match for a Series 2). But the Z1 is not terribly heavy, much lighter than an RRS BH-55 or Kirk BH-1 so it could be a good choice. I don't think it is so heavy it will stress the Gitzo, it is more a matter of that that 0.3 LB or so means to you in terms of portability.