I think you're right (now that I've spent all night thinking about it). Arduino is good if you need to do a bit of processing/decision-making etc.
I just need a chip to read an analogue value super fast and bang it out to a parallel port with minimal latency, at an arbitrary rate controlled by the parallel port strobe pin - no brains required lol.
There are soooo many chips that sound awesome for this, but then you read the datasheet and find out they can't be run at such a low rate... Cue the time machine...