Here's what the official Termignoni looks like, seems to be a underseat/underslung combination.
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Bikeman
I cant see this as anything but a track unit. It's complicated making it expensive.
Moreover, for the street, the under-tail section would require a special bracket for the license and turn signals (and/or an integrated rear lighting assembly) which Termignoni would have to provide as nobody else would make one for them.
At this point, I'm just wishing for an after-market slip-on that's more efficient, sounds good and the exhaust exits symmetrically from both sides of the bike as the stock unit currently does. IMO, this would be the most cost effective and marketable solution for most buyers.