Staintune do a good pipe, I think Laser do a pipe for the B12. Both of these have a small advantage over other pipes in that they are designed to 'convert' from ADR Compliant to 'For Race Use Only' and back again by simply undoing a screw and taking out part in the internals. No more having to go home - put the original pipes back on - getting the defect cleared, then going back home and taking them off again. When pulled over simply say
"Crap, sorry officer, that bloody thing keeps coming out, thank Christ I carry a spare one with me......"
Now, I don't have such a pipe (The Treasures bike takes precedance when it comes to go fast bits) but knowing how dumb some of the 'boys in blue' are they'd probably buy it....
Of course Yoshimura, and Akropavic, Remus etc etc. There's a pipe from the UK (I think) called ART that fits the B12. I haven't heard one on an in-line 4, but it sounds pretty wicked on an Aprilia Tuono