After cross testing with GNU/Linux, i've found out that my Intel 82577LC em(4) ethernet adapter won't do gigabit ethernet even when forced to with media options on #OpenBSD *head scratching*
@canadianbryan I know why if_em.c use auto-negociation no matter what now, thanks :)
Sorry if it wasn't clear, but the exact same setup does gigabit ethernet if the computer with the "faulty" adapter runs GNU/Linux. I tried with 3 different cables and they say the same story. Throughput also match with the media type used.