I've spoken to Dietmar myself at Luftwaffe Experten, might be worth logging in there occasionally to see if he pops in. Gave me some good info on the Ta152C I was researching, and was messaged some handy datasheets and excerpts of various publications by other members (they were terrifically helpful). I wouldn't have been able to accurately remodel the type for a flight sim without them and am extremely pleased with the result (pretty sure I got a prototype version without the wing tanks using C3, a preproduction series with wing tanks but the fuselage tanks removed using B4, and the zerstörer version all pretty spot on for handling and performance).
Having some engine building experience I use a pragmatic approach to researching actual service performance in models, generally I find there is enough conflicting data to discount point matter which is relatively isolated and I consider mechanically unsound. I find some wartime report notations ambiguous and always seek to find context for each point.
For example, with the Jumo 213E wrt to GM-1 the overriding factor was an entirely unreliable operation of the third gear of the supercharger when used in conjunction with the second stage at altitude exceeding 10500 metres, which to be honest I currently consider to be the actual in service flight ceiling for the type. From what I've been able to determine, what happened was the second stage would keep bumping the supercharger back to the intermediate gear. To be honest it would've been best if the automatic (pressure operated) gear speeds were only for the low gears and the high gear was manually selected like in American planes. I should mention this is in part speculation but I'm pretty certain of it and it fits well with reports and data, and explains the disparity between projected performance figures and test results for the 213E.
It is true that two (?) flight tests exceeded 12000 metres but both were dicey occasions (on one the pilot lost consciousness) and the only time a height of the 14500 metre projected maximum ceiling was achieved was by Kurt Tank himself, no other pilot managed it. Following initial testing the ceiling of 10500 metres for safe operation was muted largely for reasons of an unreliable cockpit pressurisation system, which was at a rudimentary stage of development.
In any case from what I've read 9500 metres is the best maximum performance height for the type, for reasons of unreliable high gear operation. This is the main factor which makes the DB603L a better high altitude engine, since its best maximum performance height is 10500 metres at which it is completely reliable.
Okay so now firstly GM-1 doesn't do anything for you until 12500 metres, at which point it gives a short boost for each gear (ideally) at 1000 metre intervals, ostensibly to return throttle height performance at 12500, 13500 and 14500 metres.
But what actually happened was it being a drawn out process to get the Ta152H to 12500 metres for the first boost, which you'd use to get you to 13500 metres and that's about all you've got, with an unreliable cockpit pressure system (your blood trying to boil in its veins).
A further complication is where using MW50, at least in flight testing using the system it had a max usable height of 6800 metres again due to unreliable operation of the high gear. Go higher altitude than this and use MW50 and the supercharger would assume you were at a lower altitude and kick in the lower gear. Switch it off and it'd bump back up a gear.
So reliable operation of MW50 is only in the first and second gears with best gains below 5200 metres.
From what I've read MW50 and GM-1 were both fitted to preproduction, but not prototype Ta152H. The sole reason for this was there was no wing tanks in the prototypes, and the MW50 occupied the left inner wing tank. But in service there would be no opportunity to use the GM-1, particularly considering combat engagement heights were around 7000 metres from what I've read. And for the preproduction series you wouldn't use MW50 above about 5500 metres anyway, it wouldn't do anything for you.
But I'm pretty sure GM-1 tanks were fitted and filled, for reasons of correctly weighting the airframe at the very least.
Also it might be a failing memory but I though sondernot was 1.78ata (1.45ata start und not).
Most of my research was about the Ta152C however. I can be readily pulled up about any point I've made here.