The main problem you’re going to have with 6speed & higher auto-boxes is the lack of aftermarket controllers for them - as they’re all electronic. That’s no problem for a number of the 4 speed auto boxes, and yes, 6 speed controllers are in development, but I don’t think there are any truly available or proven yet. (Meanwhile mgrs have moved on to 7 to 10 speed autoboxes - all with proprietary controllers & proprietary firmware).
I just saw LongJohn’s post… I see USShift has finally put that controller on offer instead of still in development… so there’s one…
Cracking/reusing a mgr’s trans controller is probably possible, but I haven’t seen much indication that there’s any widespread knowledge on making that happen with any of the OEM boxes (although some high-end tuners seem to have $$$erd their way through it in some instances - not Jaguar specific.).
It’s a nice idea, but it’s going to take figuring out who knows how to adapt the trans control boxes - which usually need like - CANBUS input from the ECU - which the XJS doesn’t have.