Yes, thats a good and legal way to do things.
Good luck to anyone who tries to make it vent by free - revving it, so you won't fail on noise. And since its' behind the passenger compartment, its legal in that respect too.
I've seen a couple of cars done that way, it sounds quite cool. From inside the car it sounds tough, from outside it just sounds feral (wether you like that or not is another story).
One of the exhaust shops down here refuses to do externals venting downwards, they will run the pipe to the back of the car however.
For extra cool points, I suggest having the wastegate pipe run out the bonnet, and up the "a" pillar. At the top of it, have a little flapper on it, like a truck. That way you don't need a boost gauge - just watch how much the flap opens through the sunroof
