I think we are running up against a dictionary definition of "all new" vs. "evolutionary".
The 2.5 turbo is an evolution of previous Subaru motors. In some ways it is definitely brand new - there was no 2.5-litre turbo factory motor before. In some ways it's merely evolutionary - much of the design isn't going to differ greatly from previous designs used in their flat fours.
Technically, the current JDM 2.0-turbo isn't terribly
different from the original, hand-made motors in the Ver. 0 STi's. However, it's obvious nearly everything has evolved and been tweaked since then.
Base line - the 2.5 turbo motor is not a "tweaked" 2.5 NA motor. It certainly is related to it, but then, Whoopi Goldberg and Halle Berry's physical makeup also may technically be 90% the same, but it's the other 10% that makes a world