Being the only RWD BG Legacy in the US (or the world to my knowledge), I pay particurlarly close attention to RWD Subarus. It's not all that "hard" to drift AWD compared to RWD (I've done both), but the style is completely different and a lot of power helps. The featured STi is also a D1GP car, which prohibits AWD layout, so to use this car in this class Kumakubo really didn't have a choice.
The rear-mounted radiator and side inlets is to get airflow through the core as the car is going sideways; think of it as the radiator equivalent of the WRC sectioned wing ;). RWD cures the inherent understeer from powered front wheels and helps realize the incredible balance that is designed into Subarus. Something else that is realized is also how inferior a lot of the rear drive components are to other cars on the market (R160<all). I'm upgrading to an R200 to increase the reliability and durability of my own car, but only because the STi R180 is far beyond my price range. I have to say that I don't necessarily like how Kumakubo did his conversion, with Hollinger transmission and Skyline rear end, but to compete in the D1GP it is a necessity to have an extremely durable and flashy car, so this was a logical route.
It's too bad magazines don't move, because it makes me a very proud Subaru guy to see this hulk of a car (when compared to Silvias, RX7s, Corollas, etc) that was designed with little intention of being 2WD completely beat up on cars that are essentially "factory drift cars". I think you guys ought to be happy that we Subaru Drifters want to use Subaru platforms rather than an Nissan S13 or S14 like soooo many other drifters.
BTW, I don't care if people think I'm an idiot for converting to RWD - you don't know what you're missing!