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Aren't most of subarus cars based on the same frame as the STi? Half of them have the STi drivetrain and/or engine optional anyway, the only disadvantage would be roll center/weight.
The current STi is actually based off the 05-09 Legacy GT chassis and the 94-01 Imprezas were based on the 93 Legacy chassis. The 02-04 were on a modified (read:stiffer) 94-01 chassis.
Subaru did make most drivetrains easily interchangable though.