It is nice to see that we all have different ways to make this flawed guitar work for us.
In the end I boldly tuned up my 09-42 string set to A after all. As already mentioned the resulting tension is quite high but it also greatly improves the volume.
The neck is still straight after 2 weeks so I think I will keep it this way.
The tuning instability with changing temperature still baffles me though. The pattern is always the same:
- I pick up the guitar and it is always flat, even though the strings are fully worn in by now.
- I tune up and start playing. The guitar appears to stay in tune reasonably well...
- ... But no. When after several minutes I check the tuning all the strings have gone sharp by roughly the same amount
- So I tune down again and continue playing. There are no more subsequent major tuning issues.
- I put the guitar away. Then it is rinse and repeat for the next session.
So it seems the warmth from pressing the guitar against my body, fingers on the neck and fretboard etc, is causing it to go out of tune. This really should not happen to a CF guitar!