this old chestnut eh!
just to back track a little first, don't forget that now it is winter, the asir is not just cooler, but as a consequence it is denser, i.e. it can hold more oxygen molecules per whatever measure you want, than warmer air, more oxygen, bigger bang, better response, more bhp, etc etc
so all internal combustion engines, if working/set correctly, will run slightly improved with the winter scenario, so don't let that complicate your testing.
right, back to bypass pipes, hmmm, they have been done before, the first i know of on a production car was Honda NSX of around 1989 vintage i believe, might have been only the type R version, but it ran flaps that allowed the exhaust gas to either pass through the whole of the silencing material, or take a shortened and less silenced path with the flap/switch thrown.
at the time it was marketed as a noise reduction device rather than performance enhancement, but would make qwuite a difference if the silencing route was substantially less, like bypass the entire silencer/muffler would be best.
i've not been asked to build one before, but i have thought about the process actually, and they're are two routes i would go.
first, would be a mechancial flap, cable operated, maybe a bit of a solid linkage to prevent the exhaust heat transmitting directly to the cable which probably wouldn't last too long,
the second route, the one i favour becuase its a bit james bond, would be the powered valve, operated by a push/pull motor and cables, on a nice dash switch, and the best thing is, the hard parts already exist, could be bought cheaply, and would require adapting.
ladies and gentlemen, introducing the Yamaha EXUP valve.
look it up, and you'll see what i mean, let me know if you think it could be integrated as a seperator.
the best thing is, the hardware sits very early in the bike pipework, which was actually quite a large diameter too, so it is well up to the temperatures that our exhaust would produce at the centre box section, or anywhere else in fact.
let me know what you think...