Ok.. my setup is 10:1 FMU, inline pump, Garrett T2, IC... running 8 psi, although hit almost 9psi at high load on higher rpms (yeah... boost creep due to internal wg).

Since I was mostly driving in town.. it all worked OK - when I step on it, all bars on A/F gauge lit up... in any gear.

So few days ago I went finally outside city on a trip.. I drove constantly 110-115mph - at which point I was at 0psi, on uphills 1-2psi. At that time A/F reads 2 bars less. But - when I then step on it, it still shows 2 bars less on A/F gauge (yeah.. it's original narrowband O2...). If I decelerate at that point, hit in 4th, and step again - same... 2 bars less shown.
After I returned home... and drove arround city (even after 1 hour of drive), and step on it - all bars lit up.

Is this normal that under constant high load (as driving constantly 110+mph), car tends to drive leaner, even when you hit fool throttle?

Besides it showing 2 bars off... everything else seemed OK..

 

Dont worry about it. As you probably already know, that gauge is worthless. It's not going to give you an accurate reading.
If it makes you feel any better, my car would do the same thing. I had it tuned for a 12:1 afr and it would be in the stoich after driving down the freeway for about 20 min. otherwise it would be in the rich under load.

 

Ic... so it's normal