Don't disable auto-negotiation, if it is unable to obtain the maximum rate is a problem with the driver, the cable, or the remote side (..switch/router).
Investigate those before forcing it, also, 1000baseTX exists but is not popular nor is it widely supported.. 1000BASE-T however is, it is possible that your interface actually supports 1000BASE-TX but perhaps none of your other network equipment does, opting to instead support the former.
It is also possible that the FreeBSD man page for
re(4) is full of shit, and they can't tell the difference.