Bought the adaptor for some other use case and just thought "lets see what happens", bummer bout that HW clipping limit.
I am currently running the iMac + 2nd 1680 screen as my MorphOS machine and it's o.k. but with the adaptor working where it is supposed to work I might shuffle around some screen and end up using the PMacG5 with 2560x1440 + FullHD setup on the 9650 I got in there.