[aerohead]

1) For all I have, I have no drag data for empirical measurements of that specific body configuration.
2) Komenda did test the 550/1500RS Spyder, with no windscreen, no driver or passenger, 100% tonneau, and no head fairing, which came out at Cd 0.247.
3) This Cd 0.247, when adjusted to Hucho's reporting for 'actual' full-scale measurements might reflect a 'true' Cd 0.3003, which is higher than the 550 RS Coupe, at an adjusted, Cd 0.2821.
4) Your 356 Speedster, replicar, with 'no windscreen,75% tonneau, driver's head protrusion excrescense into the airstream, and head fairing, would fall somewhere a bit higher than Cd 0.3003.Can't be more specific.
5) We have no specific, isolated data, for any aerodynamic contribution which can be attributed solely to the driver's head fairing.