An update.
I got my breakouts made, and soldered on the ATMega2560-8AUs.
I added a new board to boards.txt, the burnt the default bootloader using a USBTinyISP from Adafruit.
Something odd then, that maybe someone with an Arduino MEGA 2560 can help me with. With the old MEGA (the 1280), the bootloader put a program like 'blink' onto the chip, such that the 'L' LED would flash periodically. I'd best drescibe it as a 'dash-dash-dash...' sequence, if you think in morse code.
But when I put the bootloader on my reduced-speed 2560s, I get a 'dot-dash-dot-dash...' light sequence. Is this an indication that the bootloader upload didn't go quite right, or does the MEG2560 bootloader just have a different default flash sequence...?
I assumed that it was just a different flash sequence.
Next, I tried to upload my program using an FTDI Friend. I had managed to upload to the Arduino MEGA1280 using this method, but when I tried to do it on my 2560, it would sit at the 'uploading to board' status.
So then I added a new board to boards.txt that would use the USBTinyISP to upload the program. When using that, the program would upload but then there were two new issues;
The serial output is garbage now. I've tried receiving at half the baud I was expecting it to send at (and I even tried double, just to see), but it was always garbage.
Uploading the first program after burning the bootloader was fine, but if I tried to upload again then I'd get an error (something about verification - I forgot to write it down) and the program wouldn't be uplaoded.
...I know it's a lot, but any thoughts?
Tom