And here the results of the compression test:
#1 172 PSI
#2 175 PSI
#3 177 PSI
#4 180 PSI
#5 180 PSI
#6 172 PSI
#7 180 PSI
#8 175 PSI

All the spark plugs look fine, look the same and have the same gap as I set them 34'000 miles ago (see the picture).
Nothing really explains the increased fuel consumption or the lazy engine feeling.
I replaced the spark plugs, just to be on the safe side.

After this job (very time consuming in case of a F-Body) I resetted the PCM and did an Idle Relearn.

Results:
Fuel consumption - no idea
Power - feels a lot better!
Fuel trims: are a little more on the negative side than before replacing the spark plugs (?)
Maybe it's the new spark plugs, maybe it's the PCM reset. However: it is different.
I'll post about the fuel consumption later on.

Stefano