Hi, I am sorry for the delayed response.
I checked the centering of the OFDM signal and it looks pretty good. Even looking at the signal's spectrum using SDR# on other computer was very nice - book-grade example of nicely shaped OFDM spectrum. Today I upgraded from GNU Radio v 3.7.6.1 to 3.7.7.1 and reception (syncrhonization and detection) has improved dramatically. Even at relatively low sample rate (1M, anything lower simply doesn't synchronizie). Unfortunately the received packets still weren't enough for any kind of real world transmission.
Was OFDM improved in some kind of way in v3.7.7.1 (I don't see anything related to OFDM in changelog)? I simply launched the same flowgraph on newer version and it works much better.