I think I have a clua why the FAT shroud works well. Like the type 1 shroud, it guides the air left and right untill it dumps it onto the heads.
Here's a comparison of a different approach to mimic the original porsche air deflector.
Both are Remmele shrouds.
The blue shroud is actually an adaption of a Remmele shroud for an Ahnendorp machined case. However, the only 'remmele'touch is that it has a more flat (hollow actually) back side, intended to divert the air better to 2 and 4th cylinders.
The second one is an all original Remmele shroud (the carbon one). There the flat back is even more pronounced and it also has very distinct 'paths' to direct more air to 2 and 4th.
I have no conclusive data in the forms of temps on either tho. That will take time
Best regards,
Walter