Here's my take on the situation. Based on numerous U.S.-spec Japanese cars in the past (all Acura/Honda Si/SiR/Type-R vehicles, Impreza WRX, etc.), their engines are lacking in power compared to their JDM counterparts. Clearly our U.S. emissions regulations are more strict than those in Japan, so when those same engines make their way here, they have to be detuned a bit, especially since a lot of Japanese car manufacturers try to aim for the LEV certification for all of their cars when being imported to our shores.
Now onto the case of the WRX STi. Somehow, Mitsubishi pulled off sending a 271 hp/273 lb.-ft variant of the JDM Evo motor while staying within U.S. emissions regulations. In Japan, the WRX STi and Evo were neck-and-neck when it came to power production. I believe that Subaru could've easily made the 2.0-liter STi motor pass U.S. emissions test, but at the cost of competitive power, probably producing less than what the U.S.-spec Evo makes. So Subaru probably figured, "Hmm, they'll never guess we'll do this," and worked with a 2.5-liter engine. Like what all V-8 enthusiasts gloat about, big displacement has a higher potential for churning out bigger, better numbers. That way, they could easily add a lot of horsepower and torque (especially in the low- and mid-range) while still turbocharging the motor and building it to outperform the now dwarfed (power-wise) Evo VIII and passing U.S. emissions. (Clearly, if you had a little motor, say, from a 1999-2000 Honda Civic Si, and turbocharged it to have as much power as the WRX, it definitely would not pass emissions tests. But if you had a larger motor that was still turbocharged to the same output, it would pass.) I figure if Subaru had the same engine but in JDM form, it would've easily had 320 hp due to less restrictions, even though they'd still advertise it as 280 hp.
I'm no expert, but that's just what I think Subaru's reasoning for the 2.5-liter. Obviously as import enthusiasts in the U.S., we have been complaining for years how we always get shafted when it comes to Japan's top engines, so what does Subaru do? They give us an exclusive bigger engine with more horsepower and torque than any other stock STi in the world. It was either this or a detuned JDM STi, and I'm definitely not complaining with Subaru's choices.
This only creates one problem. For those of you who are into all sorts of cars (not just imports), you might have noticed how manufacturers of luxury sport sedans (Audi, BMW, Mercedes-Benz, etc.) are being VERY competitive. First Mercedes dropped a great 354 hp V-8 in their E55 AMG, then BMW threw a 394 hp V-8 in their M5. Audi wanted a piece of the same pie, so they threw in a twin-turbo 450 hp V-8 into their AWD RS6. And guess what: Mercedes dropped a supercharged 469 hp V-8 into their redesigned E55, and BMW is working on a 500 hp V-10. Basically, it will never end, and numbers will continue to stagger, which isn't a bad thing. But in the case of the U.S.-spec STi and Evo, the battle may go down a similar path. I figure Mitsubishi might up the ante for the U.S. market and throw in a curve ball of their own, probably an engine of similar dimensions in the future, especially since Subaru didn't just give Mitsubish the finger--they pretty much dickslapped them across the face in front of everyone. Who knows, depending on what drugs they're doing, you might see more cylinders in the U.S.-spec rally cars if things get more competitive (but let's hope not).