I have been searching through this forum and several others and have found that NC23 headers can be used on a NC29 and Vice versa. What is needed are for the frame mounts to be removed and new ones made.

Can anyone comment on the performance of say a NC23 4-1 system on a NC29 engine?

I know that the 4-1 are better for top end power but worse on midrange then 4-2-1 but would the 23 system still be good for the 29's top end?

The engines are similar enough that I would imagine the exhaust system dimensions are the same (I know R model are different).
Are you talking about standard Honda systems? I'm pretty sure the standard NC23 headers are 4-2-1.

I'm talking about aftermarket systems.

I believe any system designed for a Tri-arm will affect the performance of an NC29 (possibly not R model) in exactly the same way.