None of these exhausts on the you tube clips sound very good. This could be the micorphone positioning.
I fitted a CSC twin pipe rear to a mates 2.5 with the Supersprint silencerless centre pipe. It sounded much better than any of these. It retained the nice crisp fizz the 2.5 has on large throttle opening acceleration- especially above 5000rpm. It did not have the brash, flat base note that the 3.2 engines suffer from. It was on the verge of being too loud, but definitely not chav. It did produce the most incredible barritone yell though. Simply very, very nice indeed.
At idle, I would describe the sound as obvious due to its bass note. Around town, it was also noticeable. Not in a boom box Corsa way, but in a real pedigree engine sort of way. At cruising speed of about 60mph/100kph, it was noticeably vocal. Not a droning sound like a 4 cylinder car, but there was a certain bass which did enter the cabin. It was on the verge of annoyance, but not quite. It did dominate though. Funnily enough, if the car was driven faster, it actually became quieter, and it was not until a cruising speed of about 100mph that the exhaust bass would become an issue. At no point was it sore on the ears though.
The smaller twin pipes prevent the turbulence induced popping that really doesn't sound very nice on the you tube clips. Instead there is a more natural popping, a bit less slapping and more just popping. I can't think why CSC offer an oval, because in the V6 or a CF2 twinspark, it just acts as a resonnance chamber.
Overall, this was a very nice sounding set up. OK, not as spectacular as the CSC on a 75 3.0, but still a notably nice sounding car in keeping with the 156. It was always commented on by people in the club, and sounded much nicer than a GTA does. It was not expensive, but it just worked really well.
Even my BMW owning ex-business partner (say no more) rated this as the best sounding car owned by a regular customer of the garage business.
A Novitech may well sound better, but I don't know. I just know that the Supersprint centre and CSC twin rear workes very, very well on a 2.5 156.