Jeroen,
EPS Hoogezand doesn't have a stainless (or any) 914 muffler in stock. They can only produce a single piece by hand. Since they work with already available components it will very likely become a "riced out" muffler sounding like a leaking exhaust. You know, the kind those Golf GTI and Honda Civic guys love so much...
A friend of mine did have a custom made sports exhaust system from them, for one week to be exact. To his relief he was then able to buy a Supersprint system from them. Guess why?
As for the Mittelmotor muffler; not my style. No OEM appearance, large exhaust pipe diameter, and I don't like the "extra sound".
914s are noisy enough as it is, so IMPO, the quieter, the better. Besides, it is the engine that predominantly determines sound character. All you get when you desperately want a "tough sounding exhaust" for an engine that simply doesn't sound well is, you guessed it again, the leaking exhaust sound. This all changes when you have a larger volume type 4 engine, let's say 2.4 liter, then you need such a bigger exhaust.
Oh, and to what's cooking: I'm curious whether SSI has picked up on manufacturing a SS 914 muffler? The SS HE are "bulls eye", so why not a produce a muffler? That was sort of the intention of this topic...
Cheers to you too, mate!