FastSerial library from AdruPilot-Mega with TX buffer look like an option, any advice.
That probably would be a good thing to try, having sending not being a blocking function, but rather interrupt supported.
Also in addition you can look at increasing the rec buffer sizes if you have SRAM to spare. HardwareSerial.cpp file in the arduino core library is where they define the rec buffer size used, presently set to 128 bytes.
Lefty