March 31st update:
The whole system is assembled! The piggyback was updated with best-guestimation mapping, and the car fired up first try.

I ended up using all the factory heat sheilds, albiet modified to accomidate the new hardware. I'm now breaking in the new clutch and turbo with just the wastegate actuator, about 6psig of boost. The car feels defintely quicker than a stock Cooper S with 11.5psig! The Clutchmasters clutch is insane, very grippy, like a switch. I have confidence it'll be definitely strong enough in the roughest application.
A few pics of the finished engine bay: