I say +1 to all the intellectual responses. It seems that the bigger the better, since SuperTrapp is known to be restrictive unless you have as many plates as you can fit, at which point you have a pain to maintain. The typical assembly is a straight-as-possible pipeset and a canister muffler, usually 3.5 to 5.5" tip, at anywhere from 16" to 22" length, and overall diameter of 6-7". This is a legality, the muffler almost doesn't matter, it gets you past the Police without any eyebrows being raised. It also provides wnough backpressure to prevent stalling.
If you have access to a dynomometer, and have trusted records of your present tuning, changing over to a SuperTrapp will either give you very little gain, no gain, or a loss. Unless you have an OEM piece now. Tanabe, HKS, ProDrive, Perrin (top end model)... these are all almost identical, and for a reason. It is the basic underlying design which is purely functional, light-weight, and fairly attractive, if you like this sort of look.
SuperTrapps are also tremendously expensive. To build one to suit a WRX is probably around a $300 ordeal, where a carbon/Titanium HKS HyperMedallion is $269 shipped on eBay, and is a known and trusted setup for the Imprezas, no matter what model or year Subie you have. I am not saying dump the SuperTrapp, or that HKS is best... everyone has an opinon... it is a choice, and this one is yours.
Your best bet for use of the muffler you are interested in is:
SuperTrapp Polished 19"x3"inx4"out (good for turbocharged models)
SuperTrapp Polished 19"x3"inx5"out (louder, more flow, good for upgraded turbos or higher HP applications)
SuperTrapp Polished 19"x2.5"inx4"out (or 3.5"out if you want quiet over power gains, this is recommended for normally aspirated / non-turbo engines)
I recommend the polished finish... it is far easier to maintain.
For $300, that ProDrive mentioned up there is going to net you a definite gain in power, and will fit your car without a fight.
Good luck finding a local welder who isn't going to spit flux on your mirror polished $300 muffler when putting a hanger bracket on it is on your plate. I know I'd be mad as a hatter. Most of the above mufflers can be used with a WRX specific hanger clamp, or they come with a hanger robotically welded, and will pose no problems.
I will also say that since this question made it to this forum, and it is a matter of "which t-shirt looks best with these jeans?", you can try it, take a ton of photos, and some clips for You to the Tube, and we might all want one. That's how most systems get popular. That and being passed by someone with whatever setup.
Also, don't have Joe Welder do it. find a real exhaust shop that builds hot-rods and track racing machines, and pay the extra labor to have strong, rustproof coinstacks of weld, rahter than the "Muffler King" spatter and braze, with the potential for a hole shot through it by the torch (most of this stainless steel is thin to keep light weight). They'll turn the muffler over after tacking the hole with braze, leaving a rough spot in the airflow where it matters.
If it was my $300, I'd get a canister w/ a really sweet billet hanger, a complete turboback, like a TurboXS or Tanabe. One with a whole blueprint, and T-304 (or higher) Stainless from the turbo to the tip.
Yeah, realized after I posted a freakin' book... I was researching the SuperTrapp for my WRX EJ20, and came across this, thought it was full of garbage, and that a real response would at least justify keeping it in the archives.
So... did anyone ever try this? It was designed for motorcycles, and a few Porsche owners went this way, simply to shut up the very short and extremely loud exhaust systems available in their aftermarket.