I've never had forced induction (except one afternoon when a BMW dealer gave me a turbo 4-cyl 1-series loaner). But I've never heard of people having issues with this kind of setup running low boost for street driving, and I don't think +4 or +6psi comes with or needs an intercooler. If you take it on the track, you might think about adding an oil cooler or upgraded brakes, depending how hard you drive it and whether you do Thunderhill in November or Buttonwillow in July. This kit will add a little extra weight and can make some engine maintenance more work as anything that previously said "remove air box" now turns into "remove air filter, supercharger belt and blower." Gas mileage may be affected!
Now for the actual downsides--basicly what Minalmo said.
- You could embarass your friends with Mustang GT, E46M3, Camaro, etc. Is a fast car worth the price of lost friendships?
- As soon as you pass smog, local CHPies will ticket you, not for emissions, but for speeding!
- Your wife/girlfriend/roommate/Mom will insist on borrowing your car to go to the supermarket
- You will become obsessed and blow your tax refund and trust fund on track days
- You'll become addicted to boost, leading to intercooler, then oil cooler, more boost, new tune, headers, exhaust, and eventually methanol injection.
- You'll feel the need for a LSD then smoke your rear tires all over town, needing new ones every 6 months
- Klingons and Vulcans become insanely jealous, leading to all-out war in the neutral zone