ACT do track day cans that slip on the tail pipes reducing the noise further by 8-10db. I have seen systems with bolt on replaceable tailpipes. Also you can slip the cats back in and you get anther 8-10db if needed.

Mine is 106db decatted at 4500+ revs, or I've managed between 96 & 98db with the trackday cans on, depending on who's measuring. At normal driving it's quite refined.

If helps

I have in my garage back box for Cerbera speed 6 2003

I took it of some now four years ago

I fitted new sports box ( too loud if honest ) then sold the car

I would have no idea what's it worth but if you are interested , I can make available etc

I will check to see if I can find an ideal guide when it comes to it's value

If wanted just let me know

Andy ( sadly misses his Cerbera )
Drives a griff now a days great but not a Cerbera

The pipes that connect to the intermediate pipes were about 150mm short and were closer together on the SP6 box that I put on my 4.2

I have just taken mine off to weld on some stainless sections to make it all fit properly after a few years with it bodged on with a couple of short lengths of mild steel pipe between the back box and intermediate pipes.

If you want to quieten your 4.2 for track days, I run a linked decat and standard (SP6) back box and have regularly passed 101dB noise tests at sprints. The noise measurement seem to vary from track to track and the marshals have told me that it measures variously between 95 and 100 dBA depending on which track it is!

I've seen bolt on ones as well: