I don't know about everyone else. But when I hear 'that was 800 years ago' I believe they are referring to it being the year 800, since it just so happens that the story begins 800 years after the first sleeper ship arrived in Sirius.. soo..
I thought it was implied that the 'Sirius System', which is what is stated in the opening movie for the campaign, either meant the star or the general direction that Sirius was located towards in relation to the Sun, which is opposite the galactic core. While I won't discount the fact that there is a possibility that your calculations are correct, and it's pretty good math given what we have to work with, it always seems to me that Sirius Sector meant Sirius, given the name of the star, the fair chance of encountering a cluster beyond it, and the fact that the New York star, the center of the sector in-game, is a 'Medium White' which ties well into Sirius' role as a Class A star, which are all typically white in appearance to the human eye.
Threads like these pique my interest, even if I don't respond to them often. I love seeing people poking the intricacies of the Freelancer universe, and even just math is clause for debate and curiosity.