My experience with Xbee S1 modules is that they're only reliable at 115200 in clean environments. These modules operate in the 2.4ghz band. If they're in close proximity to any sort of wifi router that also operates in the 2.4ghz band, then 115200baud even at short ranges was unreliable for me.
I've had no problem working at 57600 baud though, even in close proximity to wifi routers, and by close proximity I mean a wifi router sitting mere inches from the Xbee on an RC rover.
At 57600 baud, you get a theoretical max throughput of 5760 characters per second (there's a minimum of 10 bits per character. 8 data bits, but also 1 start and 1 stop bit). With an 85 character data packet, this will provide a maximum theoretical update rate of about 67hz. 50hz should be a real world attainable rate though.
85 bytes every 50ms, it's means that I send 85*(1000/50)=1700bps so with 3500bps connexion it should be ok ?
Your data rate is whatever baud rate you are operating at. Periods of inactivity are irrelevant in that respect as, when you send 85 bytes of data, it is being sent at the baud rate specified. From a transmission point of view, there is no credit for periods of inactivity. That's not to be confused with any processing time requirements on the backend or frontend, which may or may not benefit from periods of inactivity.