personally i see little need to go beyond 57kbaud which is standard for most of the arduinos out there. even 115k was not worth the hassles let alone 1m. cable capacitance, divisor error, os problems, etc override download time issues for me.
speaking of which im surprised to hear things like 29% improvement of 1m over 115k and that serial is faster than isp. flash page programming time is the real bottleneck. although ive not been able to try 1m for reasons mentioned, in my tests 115k was only marginally faster than 57k and no difference at all for 230k. same results as at the freak site couple years back. and to rule out silabs which i find not only cheaper than ftdi but more trouble free clinches it. i would like to see a no-led version for 1284 like posted in the other thread but 57k.