Default baudrate of EM406a GPS device is 4800 baud. Then this baud rate dictates entire communication of serial port on ArduPilot - also those for Serial OUT (telemetry).

Here's simple way how to permanently increase baud rate of EM 406. Once done, the device will remember the baudrate, and you can increase serial speed in ArduPilot code.

1) Temporarily connect your GPS directly to FTDI cable:

Pin connection:
FDTI GPS
black <-> 5 (from right)
red <-> 1
orange <-> 3
yellow <-> 4
Note: I used original half-cut cable + connector left by this cable modification. No soldering, just careful wire plugging into FTDI connector.

2) Connect FTDI cable, and check if GPS works in RealTerm using default baudrate 4800:

3) Switch to Send tab in RealTerm:

and
1) type command $PSRF100,1,19200,8,1,0*38 in marked field
2) check +CR +LF line-ending
3) click "Send ASCII
This will configure your GPS device for 19200 baud rate.
I didn't test with higher baud rates, but if you change it, you'll need to recalculate command checksum (*38 at end), otherwise GPS won't accept the command.

4) Now GPS start to send at 19200 baud, so you'll see some garbage in RealTerm. Go to Port tab, close link (untick Open button), change baudrate to 19200, and Open link again. If you see valid GPS data, then all is OK:

5) In ArduPilot code, search for line Serial.begin(4800);
and change it to Serial.begin(19200);
Done! You have now faster serial communication in your ArduPilot code. GPS device will remember the baud speed as it has internal battery and memory and keeps the setting.
You're now prepared for more data to be sent by telemetry link.