I suppose the correct terminology for the part that I require is a heat deflector rather than an extension pipe.
If I'm wrong though - please do let me know
Both serve the same purpose......it's not the EXHAUST heat that melts the pannier, it's the exhaust BLOW itself.
I had one of those deflectors a long while back.....bloody useless it was....ended up sticking an extension pipe on still to take the blowing exhaust right out at the back.
I think Timotei is right.....the Sebring thingy was mainly to stop the LH indicator from melting when you free up the 'zorst flow a bit, by deflecting the flow down a little.
Your alternative by the way is to use a lowering kit....WUDO used to do one, but it was silly money.
You can get flexible clamp on exhaust sections though.....a six inch piece should suffice......put it inline BEFORE the can itself and it may even take the exit from the can far enough back to avoid the boxes altogether.....being flexible, it also means you can bend it up, down, inwards or outwards so you could run the can horizontally UNDER the pannier (I did that for a while but after I smashed two cans off-roading, gave it up

)
You would need to modify or make a bracket, but honestly, that's a ten minute job with a bit of plate and a drill.
Last option.....if you still have your CAT, you can bung that back on and run a stubby (Halfords sell an ideal lump of boy racer pipe for the stubby) and then the exhaust jets DOWN before it even gets to the pannier area......From what you've said about the noise aspect though, I suspect you wouldn't like that.....it is louder than standard (obviously, as it's 'standard' minus the silencer' but still well inside limits, and the flow is a lot better than with the end can (although not as free as a Y piece with Remus and no baffle in)