I thought this was already fixed.....(because I fixed it, I believe)
In actual fpc trunk (40937), the baudrates for cpuarm not changed. I am on fpc fixes32.
But i think higher bausrates did not work out of the box. If i read the RasPi Forums, you have to use resistors (33 Ohm) and to deal with the cables for higher baudrates. So it have to fixed in synapse.
Edit:
http://fw.hardijzer.nl/?p=138 looks like the baud rate is/was limited to 187500 baud. But you have to fiddle with the kernelinformation and more. See the link from 2012.