you can only swap out with STI headlight housing or WRX with HID headlight housings (imprezas come with halogen as do WRX without the HID options). Otherwise, swapping to a halogen WRX or Impreza housing just for the black chrome is a major downgrade.
Upgrading a halogen to HID requires upgrading the projectors for HID's as well, and aftermarket "blacked out" headlights are all cheap and not worth the money. I've been warned not to go that route, secondly, most ebay aftermarket blacked out STI headlights with HID's dont have HID projectors in them, just the bulbs. once again a downgrade (you'll blind oncoming traffic!).
Your best option is to either:
a) find someone (most likely with a silver 2011 STI) to trade the chrome for their black
b) buy a used pair of 2011 headlights (prices ranges from 400 and up for a pair last i've seen on ebay) - bads is it may be scratched or will have 2-3 years of it being pelted by rocks
c) send your headlights to lightwerkz, they're in NJ and do an awesome job of recreating the stock look (its not 100%, but i love the look none-the-less - the match the black chrome really well)
d) find someone local to do the baking and blacking out for you. you may nto get that black chrome finish.
I ended up going the lightwerkz route since i wanted angel eyes and stock OEM black chrome look. ended up riding my motorcycle for 2 weeks while they work on it. In the end, i was much happier with this result than if i went aftermarket or open it myself and blacked it out (may mess it up in the process), or buying a used pair that's already got 2-3 years of chips in it for more than the cost of just having it repainted by lightwerkz. I know they charge more than if u just rattle can it yourself (lots more) - but they do a good job.
this is the job they did:
Albert’s 2013 STi – Custom Painted Housings + LED Angel Eyes | HID Projector Headlight Retrofit for Subaru, Honda, BMW, Scion and More | LightWerkz.net
the specs you see when the lights are on are all the chips i have
and that's just 1 year
you'll notice that its not exactly stock oem. the oem chrom extends a bit further. I still luvs it. i didnt want the total blacked out look and they did just fine with me. u can check out other STI's they've done on their webpage.