Posted this on another forum. Looking for thoughts here:

Warning the following comments are not that of a professional
True duals have been made for inline sixes for ages. The Old Chevy 250 being the one I'm familiar with. Also had them on a girlfriends Ford Maverick, a 200 (it's been 27 years so it's a little fuzzy). Worked grreat.
The most important design part was making sure the exhaust pulses were sequenced correctly so that each pulse helped scavenge the gases from the pulse after it. This helped purge the combustion chamber for the coming intake charge.
In practice headers for the Mini shouldn't be any different than headers for a 4 cylinder boxer type motor. Split the tubes into 2 seperate collectors, into 2 small monolithic cats then out. Tube diameter could be reduced based on cross sectional flow requirements. I.E. 2 small tubes flow better than one large one.
Even better you'd have 2 segregated pulses at the tail pipes. Then you could get the true harmonic sound you get with duals at certain RPM ranges.Would sound Bad *SS.
Adjusting collector length will also allow you to dial in the torque range too.
The preceding verbage is hot rod dreamer cloud talk but.... I'd sure like to see it
Any developers that do this owe me a set