I was reading the Family guide, as well as the F5529 guide and could not really determine how power consumption is affected by the UART in Asynch mode. I expect that baud will certainly play a role, as the core voltage must be increased to run at higher frequencies.
The thought behind all of this is driven by wanting to keep battery life at a maximum, of course...
My GPS/Iridium Satellite comm device is following this basic flow:
loop()
switch(currentState)
case ENABLE_GPS:
{
Serial1.begin(9600);
digitalWrite(GPS_SleepPin, HIGH); //enable the GPS
currentState = WAIT_FOR_GPS_FIX