I did two long road trips on the GZ back in 2006 and 2007. One was from Dalton to Franklin, GA. which is a 3 hour trip.
The other was from Dalton, GA to Dalhonega and then into Gatlinburgh. I don't remember how long it took, but it was a good haul.
Both trips were planned using larger highways, although no interstates, and some faster backroads. We maintained speeds in excess of 65 for the majority of our travels and had a great time. There were two of us on both of these trips and we both rode GZ 250s.
If the bike is in a good state of tune, it can handle it. These things aren't so fragile that you have to putt around at 45 mph all day. I wouldn't jump on I-75 and goose it at 80 for 6 hours straight. But I also wouldn't fear an arbitrary speed-limited number.
Franklin, GA meet pics:
viewtopic.php?f=2&t=558&hilit=Franklin%2C+GA.+meet
