Used my car with and without a Supertrap and although I didn't do a rolling road check, fitted wrongly they'll loose you a lot of power, but fitted and tuned correctly you won't see any difference at all. Some claim you can get a power increase, but I think that's on road stuff where the standard system is too restrictive.
Anyway, back to the real world. There are two basic types of Supertrapp, the simple end units and the full silencer type. I'd suggest the former is fine if you are adding to an existing back box, but personally I would go for the full silencer type instead of any boxes in the system. They are fully repackable silencers in their own right and are designed to work with the plated exit system. I resorted to the latter and effectively cured a nasty noise problem.
The first thing to note is that with both types, just looking at them, appear very restrictive, but that's actually just an illusion. Each plate is formed so that the gap between is the same as the thickness of the plates. At (from memory) 12-13 plates, the open area between the plates is equal to the area of the hole down the centre. So if you have a 2 1/2" supertrap, that many plates gives you the equivalent open area as having no Supertrapp fitted. Obviously because the gas has to turn 90deg there is effective resistance, but by the time you get to around 15 plates, even this is minimal and you have approximately an open pipe. It also means the noise you make is pretty much as if it wasn't fitted, but it does spread in a different way, so may increase or decrease a noise test depending on how its measured.
This gives a very effective way to "tune" your silencing down to just enough resistance to meet any given noise requirement. It's also a really neat way of meeting different requirements at different tracks by changing the number of plates fitted.
On that point, I said they can be badly fitted, because when I bought mine it came with something like 5 plates. I fitted it and the car was flat as a pancake. Then I investigated properly, bought a ten plate extension pack, put 7 of them in, and hey presto noise solved and car back to normal power.
Two last comments. First, I've seen people fit them like I did, then take the end plate off to get their power back.... err... can't see why you'd bother keeping the rest without the end... sort of defeats having one fitted. Second, even more investigations (I've done quite a bit
) suggested that silencer
volume is the key, not backpressure. So I modified the bodywork and swapped out my 5" diameter 18" long Supertrapp for a 6" diameter 24" long Merlin straight through box, with no baffles or plates at all, and achieved the same noise level with much lower restriction.
I also know a bloke who could get 4.2 litre Jag saloons through a noise test with a straight through 3" bore system... with just the one Leyland Roadrunner Truck silencer box in the middle