You may see a slight faster loading time with a SATA drive for large games, but otherwise no, not really a gain in performance.
Also, it sounds like your slave drive is on it's own channel, so there's absolutely no loss in performance there, I would keep the set up just like it is if your slave is big enough. If you need to upgrade for more space, sure, shoot for a SATA drive, but just don't upgrade purely for the sake of upgrading.