Off the shelf is OK, [A15 clutch for Vanette] But if you are chasing performance, I would recommend going for a custom clutch. No real need to larger than the std dia. 180mm is fine for A-series IMO. Its all about the friction co-efficent and the wise choice of driven plate and pressure plate. Ceramic puck [3 & 4 puck] seem to hold up ok
Also the thickness of the flywheel will impact on the heat transfer of dissipating heat generated by the clutch when using a lightened flywheel. Its imprtant that the flywheel is lightened in the right spots.

Turboing the A-series should see a significant increase in the torque generated. I would be more concerned about the transmission holding up. 56 series gearboxes dont seem to respond very well when lots of torque is put through them.