Most of the people I know love their cars and race them in many forms of racing and competition.
Making them faster is a sickness and an expensive one.
There aren't many cheap ways to improve today's cars.
I've been driving the smart almost exclusively the past few months only using the old car to tow a trailer.
Yesterday, I used the big car to go on a short trip to pick up some things that would not fit in the smart so it was empty with no trailer.
WOW! Is it a Rocket!
So my advice is, when the STi feels slow and you start thinking about a different intercooler or a bigger turbo, put it away a couple months and only drive the smart.
When you finally open the door and let the beast out, it'll be a new thrill all over again. Saving you big bucks!