I haven't posted in awhile -- but Bump!
I'm in no way affiliated with RR -- but this is an awesome price/deal. WRX/STI owners should jump on this. I would too if I didn't have an aluminum aftermarket rad already (not Koyo though) which I bought for well over the price RR is offering. The cooling system is an often overlooked/underrated mod.
Going from stock to aftermarket - my experience has been that your coolant temp won't necessarily be lower (correct me if I'm wrong RR) unless you change your thermostat. But --
-- your fans will come on a lot less to maintain the nominal ~90C temp,
-- your coolant temps will be much more consistent/steady (it may take a tad longer to warm up in the winter time though)
-- your rad will be more efficient -- so it will seem to make better use of any airflow to it. (My coolant temp easily drops from 90+ to 89/88 cruising at 50-60k and down to 86/87 at highway speeds.)
-- Because of the way subaru designed the coolant/oil systems -- there is the added bonus that a more efficient rad will help to lower your oil temps sometimes too.
If for nothing else -- upgrading your rad is a proactive measure -- because the WRX/STI radiators are prone to slow leaks (sometimes hard to detect/notice) where the plastic endtanks are crimped/epoxied to the metal core (I know mine did). Usually this happens just as your warranty runs out too!