Well I would investigate a bit more. I tried this sketch on my Mega2560:
#include <SoftwareSerial.h>
SoftwareSerial foo (10, 11); // Rx, Tx
void setup ()
{
foo.begin (115200);
}
void loop ()
{
foo.print ("Hello, world.");
delay (200);
}
And checked with the logic analyzer:
Seems OK to me.