Grant:
Whoopsy:

Double the cylinder count and displacement increases the engine's friction too. 

 No reason it would make more than double the friction (probably less that double, given only one crankshaft) - no reason it couldn't make twice the power in the same state of tune, unless there is insufficient strength in the crankcase or other component.

Simply due to the lower turbo boost.  You are correct that with one crankshaft and equal boost the 8 will be more than double the 4.  AND YES - DRY SUMP is a BIG DEAL!  Great observation!  This is why we do not see 991 GT3 engine in the CUP cars.