I bought a s/h LGU that was running c. 10.6. I had reason to believe it had been back to Armex because of low power. TP was 2.6. I opened it out to 3.2mm and with a small preload washer it went to c. 11.5, if I remember right. I then left the rifle untouched for about 2 months (uncocked before you ask) and power had dropped to about 10.5. I had bought a new kit from Germany so fitted the new spring. Output immediately regained 11.5. Rechecked two months later ... still 11.5.
I came to the conclusion that there were some duff Walther springs around.
Cheers, Phil