I've owned an AP600E GTO mount upgraded to CP3 for around 10 years and used it exclusively for close to 4 years as one of my first mounts. I still have this mount.
I have used this mount to carry an AP130 starfire and also an 8" RC. I believe it could easily handle much more. I only used the mount guided.
The only issue I saw with this mount was a bit of stiction in the DEC axis. This is a very simple issue to fix -- just guide DEC in one direction only. It is pretty easy to figure out which direction the drift is in.
Within its limits it appeared to perform about equal to a mach1 that my friend was using. The mach1 seems more robust and can carry a larger payload.
The AP900 was a definite upgrade for me as I also started using a 12" telescope for imaging.