Too my understanding the 4G63 has more potential in the BIG hp end of things due mostly to it's closed deck design vs. open deck for WRX ej20 vs. semi-closed deck for STi ej20.
So that's one for the 4G63.
Then there is the inherent balance of the flat style boxer engine that the inline 4G63 does not have.
So that's one for the ej20.
Those are about the two biggest things I can think of right off the top of my head. I still think, and this is probably becuase the DSM's have been around for 10years now, that if you take a fixed amount of money say $5k and put it into a DSM and a WRX the DSM will still be faster at the drag strip.
I only say this becuase I personally know of 3 DSM's here at altitude that run deep into the 12's and that is off the Nitrous. Where as the fastest WRX I know of here at altitude is running a 13.2 right now... soon to be taken over by me hopefully

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But just like the WRX's have a pretty proven formula for 12's in the 1/4 right now... the DSM's have things like the "green turbo" and such for formulas to get them into the 11's and 10's.
Just give the WRX time though I am sure it will be there soon enough.
Cheers