I like the jts headers for the a12 but my 1608cc a15 quad mikuni setup loved
the 4-1 headers with 2 inch system, a hotdog and a supertrapp all the way to 7500,
and still wanted to keep going like a motorbike. This is where 4 cyl 1-1.4litre
bikes have it right with their 4-1 headers. Also remember the 82mm stroke makes
more torque than the a12 and a14 in the lower ranges so 4-2-1 helps torque but
not really the best for higher rpm.
For my needs I prefer the 4-2-1 for the street in a mild setup as Im not interested
in going beyond 7000rpm these days.
Also have you looked at your ignition? as your setup looks like it should be happy
way over 7K rpm.