Hey Dan - not the technical info you're after, but you might find this idea interesting:
Low on funds, I could not afford an after market exhaust or a separate sound recording device, so I got a 2nd drift mic, a $3 mic extension lead and a $4 two into one adapter. I have one mic in my helmet and the second is in my seat bag, positioned over the exhaust & wrapped in some foam and a small towel. I run the extension from the helmet through my jacket so a short bit hangs out the bottom and allows me to plug that into the seat-bag mic (also auto-disconnects if you forget by just un-plugging itself).
The result is I can have just the helmet mic, just the exhaust mic or both together. Both together sound great, when I rev the engine it doesn't sound like the weak Honda cb500x exhaust at all!

The towel is how I "mix" the sound levels - more layers of towel = less engine noise etc. The towel also completely eliminates wind noise...
I did original try this method with the 2nd mic in my back pocket, which worked great, except when I was slow riding, like parking, it sounded like I had a severe flatulence problem