Cheesehawk you said:
"Seems though, that the supercharger effect is disappearing too quickly after 5.5k or so"
Its dropping off in the Sim at the same rate as the RAE Test data. Once you get to Full Throttle height the supercharger is effectively out of the loop in terms of maintaining any further impact on Boost drop off. Its done its job in maintaining the boost at sea level values up to FTH, after that it cant do any more....its running at 100% output. The rate of ATA/BOOST drop off is then pretty much constant ... at a similar rate to a non supercharged engine as it climbs above Sea level.
To get higher still with Sea Level ATA you would need stepped supercharger with a different set of ratios/speeds etc. Like in the later Merlins with MS and FS gears requiring a manual shift at MS gear FTH. Or the BMW801 that shifted itself via Kommandgerate output.
The DB601A was variable speed but only single stage. (I am pretty sure
. So at sea level its just idling, as altitude increases its speeds up, at FTH (5000m) its running 100%.
Good discussion makes us get into the books and references
Try the Test file I uploaded and see how you get on.