I'd just read around a bit. Rex's are odd cars in comparison to your rwd mustangs. Check out the links in people's signatures. They help.
Seems to be a common one is to throw a turbo back exhaust, accompanied by a good tune. Tuning seems required to get gains out of most mods on these cars. I don't own one yet, but been reading around for several days.
Enjoy it, just do your homework with these guys, they are weird =).
EDIT: grabbed a quick link - http://forums.nasioc.com/forums/showthread.php?t=533787 ... It is basically a list of common topics, but they are related specifically to subarus. Good thing to flip through in your time before making any purchases.