Hello,
I could be wrong but personally feel what ever different issues happened with USB.serial are not only related to buffer sizing.
As I've explained on some the threads a few weeks ago, the test i'm using is quite wide since it consists to have a Java GUI dialog with arduino mega via USB using SerPro protocol open sourced by Alvaro Lopes. In fact, this SerPro protocol is a RX/TX using an HDLC with 16 bits CRC, it has been running on different projects wether on PC or Macintosh for a few years, able to transfer one way or the other way many variables, single, frames... re-transmit if errors and so until the packets get through the link.
A while ago, i did try the protocol to my arduino DUE, in fact the way it has programmed in C and C++ is NOT dependent wether arduino DUE or arduino MEGA or arduino UNO but it never worked with DUE under IDE 1.5.1
I'm eager to have access soon to the new IDE to benchmark test if all the USB issues have been resolved.
Thank you, Albert