[SD987]

Downshifting for the purpose of slowing your car is a terrible idea unless you're playing Gran Turismo.

Rev-matching on a downshift would make sense only if you were looking for more power at higher revs in a lower gear i.e. acceleration post down-shift. If you rev-match you'll be going at a similar speed at the point after the shift, only in a lower gear, which wouldn't improve on your braking scenario. In the amount of time that it would take for you to rev-match and for the engine compression to start to slow the car you could have achieved far better deceleration simply by using the brakes. If you don't rev-match you'll have short tranny life as described by the other posters and I'd still doubt whether you couldn't have achieved the same deceleration using just the brakes (particularly in a Porsche) in the same amount of time.

Not upshifting on a down-slope (which is different than downshifting) thereby using engine compression to maintain an approximate speed or to curb the car from reaching an excessive rate of speed is an accepted practice, but then again, there aren't many slopes in Florida.