There's a JDM option for a half-sized STi wing. I don't know much more about it though...
Otherwise, you could just trunk swap with a spoiler-less WRX and buy yourself an aftermarket wing of your choice.

 

do you have a pic of the half size? that sounds cool

 

hey ill be more than happy to swap your trunk lid & wing for my factory non sti trunk and wing

 

I actually haven't seen it myself. It's supposed to be C/F and it mounts across and atop of the bottom wing mounts (In other words it replaces the top wing segment). I think it was trebsrex that told me about it...
Wonder what it'd look like if you got that and stuck it on the Chargespeed wing expander...