Are you sure? The Contax manufactured adapter I had didn't operate the Haselblad lens shutter. Shutter operation remained the preserve of the Contax focal plane shutter, so there was no change to the maximum flash synch speed.
There were repeated comments from within Contax that one or more Contax 645 leaf shutter lenses were in development (along with a second zoom lens and a Mark II body), but none of these ever materialised before the end.
it wont work- i looked into that when i had contax. it really needs a specially designed leaf shutter lens that works in this order- trigger the shutter, then close the leaf shutter, then open the focal plane shutter (for longer than the leaf shutter is open), then open the leaf for the desired exposure, close the leaf shutter, close the focal plane, then cock the leaf shutter.
the old mamiya leaf lenses worked by the aperture activating triggering the leaf lense to close, stall a litttle, then open when the focal plane shutter had the time to open.
so its quite complecated. i gave up trying to make it work (i thought i could use a dual cable), but gave up, its impossible unless you make a new lens.
those mam adaptors (also zork and another place that makes them) are for the 200 series. not sure if you can use the leaf v series with the shutter left open?