Serout & serin are limited to 38400 with all M2 parts. Picaxe Software serial ( serout/ serin) does not support baud rates that are multiples of 14400 ( 14400,28800,57600). However hardware serial can go as high as 115200 with the built in settings and can be set to a user configured baud rate by using the formula provided in Manual 2 page 95.
The problem with hardware serial on the M2 parts is the lack of a scratchpad buffer for background serial receive. All you get is a two byte circular buffer, that means you must process the serial data after every two bytes received. If hserin is used with an M2 part an Hserin command MUST be used for every byte received. This means that whatever is sending the data must put an added delay between every byte sent to compensate for the Picaxe command overhead.
You would be well advised to consider an X2 Picaxe for doing high speed two-way serial comms. However if you are only sending data, the M2 using hserout will work fine at 57600.