In this case the throughput will always be limited by the Genesis 68000, so data rates will have an upper limit. In practice it's fast enough; on the PC-Engine I could upload and run a 512K game in a matter of seconds and it is slower than the 68000.
But I wonder about switching chips. I like the FT245/FT2232 because it looks like a peripheral to the host CPU, and you just read and write it to get and send data.
I've had a lingering bug where reading data sometimes fails which normally isn't an issue as you spend most time uploading data. But in a recent project I think I've finally cornered the bug and may be able to finally get a reproducible test case I can write to FTDI about. I've heard similar complaints from other projects so I don't think it's just me.
Maybe both channels of the FT2232 would work, with one port for output and the other for input. That's assuming this problem manifests itself for mixed reads/writes (I suspect it might).
I've been using FTDI chips since 2004 and really cannot complain, but if there's something better out there that's appropriate for this project (5V compatible I/O, suitable for a host processor), I'll use it.
That said, what would make a good replacement? All the USB-enabled microcontrollers aren't really designed to interface with a host processor; this means you'd need some kind of handshaking between the two would slow things down. I think some chips like a PIC have a 8255-like parallel port with limited handshaking but most discussions of it are about the problems rather than the successes. There are very few USB products that are designed to be a peripheral for a microprocessor.
Now there are such peripherals for Ethernet, but that sounds like serious overkill in terms of functionality and cost. It would be kind of fun to have the Genesis remotely access files on a PC through a router when browsing for a game to play, or put the console on the Internet.
EDIT:
Cheaper than I thought:
http://www.sparkfun.com/commerce/produc ... ts_id=9473
Would need additional parts for 5V<->3.3V level shifting.
http://www.sparkfun.com/commerce/produc ... cts_id=200
More expensive but 5V friendly.
The FT245 and FT2232 modules are $25 and $35 so these are exactly equivalent. Now I think SparkFun has terrible quality on their breakout boards but there are no major complaints from users of these parts that I know of.
Not as convenient as USB, but it's just another option I guess.