You'd have to find a wrecked MS3 with everything working and transplant all of that into the P5 body shell. For airbag sensors, it's a fair bet the voltages will remain the same, so that should be no biggie... but you'd want absolutely everything else in there to make the ECU happy. All the wheel sensors (which means hubs and custom fabricating to match those hubs and brakes to the stock struts), most of the electricals... the dashboard gauges (custom pod it, I guess)... all the intake and exhaust sensors (goes without saying)...gets tricky if there are sensors on the body itself for the stability control... you'll have to move that module, too... oh... and the in-tank fuel pump probably won't push enough for the high pressure system, so that goes in favor of the Mazdaspeed unit...
Either he did something like that... or he's a super-genius who has created his own ultra-fast standalone ECU... or he's a semi-genius who has disabled the 2.3 DISI's DISI system and is running a regular MZR 2.3 fuel system on the motor, in which case it would have been easier to just swap in the regular 2.3, run it on a standalone, and turbo it.