I expect that Subaru's current Denso diesel ECU uses a higher-speed processor than that used in current gasoline 32-bit ECUs, because it is necessary to perform fuel injection very precisely in order to minimize particulate and NOx emissions.
According to the article at
http://techon.nikkeibp.co.jp/english/NEWS_EN/20080222/147866/, Subaru's current diesel system uses 1,800 bar pressure common fuel rail and solenoid injectors. This appears to be the Denso 5 stage injection diesel system described in the 2003 article at
http://www.densocorp-na.com/news/pr.php?Action=223. I've attached the two figures which go with that article.
In order to meet CARB emissions standards in the US, I expect that Subaru will adopt Denso's newer 2,000 bar pressure common fuel rail system with piezo injectors and 9 stage injection, described in the 2006 article at
http://wardsauto.com/ar/denso_ecu_diesel/.
Close-up high-resolution pictures of the micro-processors on the ECU's circuit board are necessary in order to identify.