I own an '02 WRX with a 'rumbling' exhaust & if it didn't sound like that at least half the fun of the car would be gone. That boxer rumble is the defining feature of a tuned WRX, it's awesome. I never get sick of it, never will.
Also, the 2.5L engine is the better engine for countries like ours & America because we are used to bigger cars with torque. The 2.0L is probably fractionally lighter & might well rev a bit higher, but (& Im speaking from experience ) it is one of the most laggy engines in the world. If you just drive them normally off a set of lights a family car will leave you for dead because you have no torque down low, & positively no power until you hit boost. Now this is something that gets old real quick!
The 2.5L doesn't have any torque issues, it still takes time to spool up but with more torque down low, it doesn't matter so much.
The "boxer rumble" is not the signature of a tuned WRX, it is the signature of a stock WRX. The rumble comes from an asymmetrical exhaust header. That makes the exhaust pulses reach the collector in an uneven pulse. This causes the typical staccato that you get from a boxer engine, similar to the single crank pin on a Harley Davidson V-Twin. When you put on a properly tuned, equal length header, as comes standard on the Spec-C's posted above, that staccato is gone. As each of the exhaust pulses reach the collector in the same interval, just like any standard inline 4 with an equal length header would produce.
The 2.5L engine is a clever way to get around emissions restrictions here in the US. The added benefit, is that it does benefit from better spooling (as a result of more exhaust volume), and slightly more torque when pulling a vacuum (i.e. not on boost). But making a comparison of the 2.5L USDM EJ25 STi motor, to the JDM and EDM spec EJ20 STi motor, to the USDM WRX motor, is really apples to oranges.
The USDM/JDM EJ20 base (non STi) WRX motor is really a lazy piece of kit. It has an open deck, mostly cast internals, no special concessions made for additional oil or coolant flow, and isnt running much boost from the factory, really only enough to make it feel fun. It is, for all intents and purposes, an economy engine.
The JDM/EDM EJ20 STi motor, is an all together different beast. Fully closed deck, forged and billet internals, improved oil and water cooling, beefier crank and rods, running a higher boost producing, higher CFM turbo. It is, at its core, an engine developed purely for production based Group-N racing. And further refined in homologation specials like the Spec-C. It is every bit as powerful, and every bit as fast spooling as the USDM EJ25. And in some cases, can even make more power.
Now the USDM EJ25 is still a great motor. But it only has a partially closed deck. It still has some cast internals to keep costs down. But at least does utilize some special water and oil cooling provisions. And because of its larger displacement, it can accept some more boost and CFM over a base WRX motor, but still not quite as much as the Closed Deck EJ20 can.
The primary reason for the increase in displacement to 2.5L in the USDM STi, wasnt so that it could be a more fun to drive, or better performing car, that was a happy accident as a result. The real reason was to bypass some EPA laws on engine CO2 output. Smaller displacement turbo engines put out a lot of CO2, and if they put out over a certain amount, they may have to qualify for gas guzzlers tax in some states, and may not qualify as 50 state legal (im looking at you California). This was a problem that the EVO-VIII ran into when it was first introduced in the US. By moving up to 2.4L, or above, the amount of CO2 allowed increases. And becomes high enough to easily pass 50 states emissions, and make the EPA happy. The same is true of a car like the Dodge Neon SRT-4, at 2.4L. It too moves into the higher emission bracket because of its displacement, making it EPA approved, and 50 state legal.
The 2.5L STi we get is a great motor, but the reality is, its still not as good as the 2.0L STi motor the rest of the world gets. Either in terms of power output, or reliability.
Hopefully you have learned something along the way. Cheers.