Silly question ... do you currently have any constraints? For example for your BUFIO2 or PLL, whichever way you are clocking your OSERDES2...
Another thing ... "So I don't know what the maximum bit rate can be." The maximum bitrate of the serdes per se is listed in the datasheet. And amusingly enough the max clock you are going to be able to pull off in a real design is a function of your constraints. ;-)
Anyways, if you haven't already .. start by constraining the main clock/bufio2/pll/whatever you use.