Just thought I'd update this (Peppernick's post reminded me).
I did some dBa tests with a sound meter the other day (DavidM was enquiring about the levels in an email to me). Thought I'd post the results here. These are the same test techniques as most trackday organisers will use (although they only measure the sound at 2/3 revs, or 4000rpm in our cars)...
All levels measured on a cold system (the system does get marginally louder when it's hot and has been on track or a long motorway run etc), 0.5m and 45 degrees from the exhaust tips, A-weighted...
Idle - 86dBa
Static @ 4000rpm - 95dBa
So the system would pass for trackday use, which may surprise some of you who've only watched the video. The exhaust is very quiet on light/mid throttle, when cruising and holding speed for example... As such, as long as you build revs fairly smoothly, then you would always pass a static test with this system.
However, as you can hear on the videos, at full throttle, the system really opens up and let's you know what's happening!

So I tried another measurement with a full-throttle blip to the redline and back... result...?
108dBa... LOL
Ooooh, I love it
