I took the Mach V STI home and ran a few errands. Hauled a big PC to work, took the kid to school. The car is still its usual self for around-town driving. The exhaust note might be a tiny bit louder at idle and low speeds, but it's hard to tell.
Full throttle is another story. When the 20G-XT-equipped STI gets up above 3500 rpm, everything starts to happen really fast, and it sings a baritone Subaru full-throttle song that turns heads -- in a good way if those listening are car fans. (I got a thumbs-up from a guy in a BMW Z3 M roadster.) But maybe that noise is not so good if you're trying to keep a low profile on the road. Not like the car in its current form is subtle -- what with the multi-vented hood, huge wheels, and all that.
The power is pretty fun. At freeway speeds you can still dip into the gas in sixth gear and get a good dose of passing speed, although there is more lag than there used to be. Dropping a gear gets the car up into insta-boost range and you'll quickly be at extra-legal speeds.
Which brings me back to the thought of losing my driver's license. I'm going to have to get the car to the track to see what it's like to drive it really hard at this power level.