If you completely swapped out the engine of the older GC8 2.5RS with a JDM V7 STI motor or even a stock USDM STI motor it would definitely be faster than a stock STI from what I remember because of the weight difference. This, as well as WRX swaps, are popular for those models, although I don't know of a whole lot of people that have done them. I'm about 90% sure that you can't just drop a turbo into the standard 2.5RS engine because it's nothing like the STI's 2.5L. You can definitely do a lot of mods to the 2.5RS engine to keep it NA and give it a decent boost in horsepower, but it's really hard to make it compete with the WRX and damn near impossible to make it ocmpete with an STI. That's why most people, like you, consider the swap on those cars.

 

Don't the STI and Wrx have just a 2.0 nowadays? I thought the 2.5RS was bigger because it doesnt have a turbo and its kinda like the "economical" wrx. Suppose to look nice like the Wrx and STI, but be as some what of a family car. Do they not make turbos for the 2.5RS? That sucks, that was gonna be my plans in the future, buy a RS for cheap and turbo it, ultimate sleeper. LOL

 

Nevermind me, I told you I dont know that much about imports, I just seen where the STI has a 2.5DOHC, as where the WRX has a 2.0DOHC, establishing why the STI makes about 75 more hp at the crank over the WRX.