Plan seems sound to me. The multi-spark, rev-limiting module sounds good. The D4F has a better bearing plate, but is otherwise the same as the D4K. Both are recurved with the same parts

Yes, it may be the carbs -- any A15 revs to at least 5600 from the factory. The D4K (matchbox) revs to 8000. However it may be the coil, I worked on one A14 that had wouldnt rev over 3000 changing the coil fixed it with no other changes (not even the condensor)