As long as they installed some reliability with a full engine build (which fully addresses ALL of the EJ's shortcomings, or just use whatever Subaru Rally Team USA is using), it should be OK.
And it has to help the center of gravity a little bit with the boxer configuration.
But I'd rather just get whatever block/setup M-Sport is using, and keep it all in the family, if I were building my own pure rally/track ride.