Hmm. I've got a street sweeper, sleeper thats been running around for years. In addition to the tips already listed, here are a few more....

Hide gauges in dash. I've got my gauges hidden below ships in the glove compartment. Very stealthy, if a little difficult to wire up.

Change badges. Easy enough.

Re-route your wastegate pipe if running external. You can have it re-circ back into your exhaust the same way you do your CBV.

Downturns. You can easily disguise a full 3" exhaust by using a downturn at the rear. By having the exhaust pipe shoot downwards, most will be hard pressed to tell what size your exhaust is. Also, spray it black. I like to use grill paint (you can find it at home depot) to paint the exhaust. Resists heat well and is cheap.

Finally, if you want a truly stock look, you can always keep the stock exhaust and simply invest in an electronic cut-out. ATP turbo stocks them.

Mod something that most won't suspect......There are tons of cars out there that can be turbo'd beyond the typical honda, mitsubishi or subaru that can go very fast. I had to start making my car look more modded because half the time people wouldn't take me seriously, just because it was so unassuming.