Likely the performance will not be hindered to much since it's really just referencing a different partition table. However the only place it would likely lower performance would be the random access time by the drive since the latency created by the software would add to that. I don't know how significant that is but even 3 to 5 milliseconds is allot. Consider 10K drives on average are only 5 milliseconds faster than 7200 drives you can make a comparison. So where does RAT come into play? When you accessing or writing to multiple files. This means using this for a cache drive is probably where you will see a drop in performance. If you use it as a media drive though, you likely wont see it unless you often have numerous video files playing simultaneously.
Thanks for the speedy and detailed response Eric!
Well, considering the system I'm currently running on the edge of what may be considered acceptable by any standard, optimisation is all important to me to achieve the smoothest workflow I can out of this setup! The latency issues you've described may be contributing to the slight glitches I'm experiencing occasionally in playback of multiple video layers with a few effects - they're not crippling but they were one reason this thought cropped up too.
Therefore, I'm gonna say screw compatibility with my Mac peers - if anything, I can always transfer data across to Mac formatted hdds as and when I need to on those occasions!
Thanks again for shedding some light on the matter.