OK, free-flowing Akra v baffled Akra v std exhaust.
I used this Android app for dB measurement:
https://play.google.com/store/apps/details?id=dmytro.com.decibelmeter2. I held the phone about 0.5m from exhaust, slightly above it
Fairly unscientific methodology, but OK for comparison tests:
1. Akra free-flowing:
idle 70dB
5,000 revs 75dB
12. Akra baffled:
idle 68dB
5,000 revs 73dB
3. Standard exhaust:
idle 68dB
5,000 revs 73dB
So the baffled Akra and the std exhaust were the same.
The 2dB difference doesn't sound (ha) like a lot, but it is when you're stood there holding the throttle at 5,000 revs.
In summary, the baffle has done it's job, and my noise problem should be solved.
I do actually need to go and ride it, but I'm fairly happy since I could tell standing there, that it was on par with the std exhaust.