I did some experiments today with a universal 4" silencer behind my current decat pipe. It certainly sounded louder. This was without any kind of tailpipe so probably "worst case". It also sounded more "interesting" with the pops and crackles more marked.
Things got interesting though when I tried to measure the sound level with a mobile app. I was measuring 50cm from the outlet at 45 degrees, 5600rpm (three quarters of maximum) static.
First I measured using my own phone. Old 6" exhaust, 86.5dB. New 4" silencer 88dB. Only 1.5dB difference.
The absolute numbers seemed low, so I used another phone, different brand, different app. Similar results, even a bit lower, 84.5dB and 86dB, but again a 1.5dB difference.
Suspecting that the microphones in these phones might be just overloaded and saturated I then measured a few metres away. As expected the numbers were a bit lower, but the difference was again 1.5dB.
I'm going to check it with a decent calibrated decibel meter so I know objectively how my noise levels compare to other people's.
SV VVC 170 - 170.4 bhp @ 7100 rpm - 142.4 ft.lb. @ 4900 rpm
I've got a couple of options for you, both from a 1997 1.6K Superlight with a long primary 4-1 race system.
1) Carbon Components Silencer
The carbon sleeve is damaged (both on the side and underneath) and needs replacing. The head of at least one of the caphead screws has been ground flat and there is a slight flat on the end of the exit pipe where it has grounded. I can send more photos if this would be of interest. This was a lot quieter than option 2) but if you get a new sleeve the I guess you could decide how much packing to use and/or get an internal sleeve to cover some of the the inner perforated tube.
2) Original 4" Caterham Silencer With Heat Shield
This is undamaged, but 23+ years / 40K+ miles old and non-repackable so it is loud i.e. has failed drive-by at Bedford and Donington trackdays and a 105dBA static test at Goodwood. It does sound great on a K Series, though. I'd only want to sell this as part of a complete system with the 4-1 race primaries which would make it the more expensive option, especially as I'm guessing you don't need the other bits!
I've PMd you as well - happy to discuss further.
The can on the 4" silencer is 56cm long and it's 77.5cm from the end of the inlet pipe to the end of the can.