An STi puts about 230-240hp to the wheels. The MS3 should be putting around 240-250 to the wheels. i.e. the MS3 actually has the higher effective horsepower.
The problem is, the STi's *massive* traction advantage... not just the AWD, but larger tires and brakes... makes it the faster car in pretty much any racing scenarios. The quarter mile is likely too short for the MS3 to overcome the STi's launch advantage, and on anything with turns (even though the MS3 probably handles "better") the STi's brute force grip advantage and ease of control, not to mention that AWD exit from all the corners, again makes it the faster car.
Really, the only type of racing that the MS3 has the advantage, is at straight line "freeway racing", which frankly isn't racing... it's just stupidity.
That said, all the stuff that makes the STi the faster car is all stuff that makes virtually no difference unless you're at the race track. You can never really use all that extra grip on the streets unless you drive like you've got a death wish. So realistically, the MS3 and the STi are equivalent, with the MS3 costing a whopping $15k out the door cheaper (not to mention it's better looking, and has a nicer interior).
I'm about as big a Subaru fan-boy as they come... and I'm planning on getting the MS3 instead of an STi for my next car. Subaru really should be a little embarrassed that the STi is so little car for the money when Mazda's raised the bar so high with their Mazdaspeed cars. The STi really needs another 50hp, or about an $8000 lower price. Heaven help them if Mazda ever puts a decent AWD drivetrain in the MS3.