If you want a Subaru sleeper, go for a Forester 2.5XT. Its a WRX engine in a Forester. The thing is probably the fastest SUV if you look at it from a speed per dollar perspective. I know the Cayenne is likely faster in a straight line but its something like $80k for the Turbo, and you're getting a 5000 lb vehicle. Kinda hard to make one of them nimble. The Forester runs 0-60 in a little over 5 seconds, well under 6 seconds. With a small lowering kit (no more than an inch), a different pulley on the turbo for more boost and a sprayer on the intercooler, you could have a very fast and nimble little vehicle. Kinda like a tall-roof WRX.
As far as getting more power from the econo-rockets, its not that hard really. Since most are turbo'd you can add a bigger turbo, or smaller(?) pulley for more boost, a better intercooler, or reprogram the ECU, plus all the normal stuff. Mopar sells Stage 1, 2, and 3 kits for the SRT4 that'll bring it up to somewhere around 300-330 I think. There's lots of 400hp Evos and STis running around. I agree that there's no replacement for displacement, but it depends what you're after. Straight power, V8 all the way. But if you want something that's just plain nimble and fast, you want smaller engines. Less weight up front=faster turn-in. That's why the Vette LS1/LS2 is all aluminum.
PS:in Evo vs STi...The STi is faster and pulls harder out of corners. The Evo carries more speed into corners but just slightly behind comin out. I read a comparison of their AWD systems and the Evo is usually running around as a FWD car until the rear end is needed, then it can split the power up to 50/50 f/r. The STi is always AWD usually at 50/50 but can change the split to be 100/0 or 0/100 I think, depending on what's needed. It might be split 40/60 f/r normally, I can't remember. But after reading that little article it made me change my mind where my $30,000 would go if I could buy either. I like the STi's setup better. Plus its usually labeled the more stable and easier to drive fast of the two.
). I should say it's hard to beat a V8, wherever it's from. There's just twice as much to work with there. In the case of road racing and rally cars, that's a different story because handling with the lighter motor and car makes a big difference.