On my mechanicals, there is only one position on the linkage that will allow for the full opening of the outboards. Looking at the picture of your linkage and the position of your outboard rod stop, you may not be getting full outboard opening?

My center carb is the only source of idle fuel/air. There is nothing on the outboards that provides for idle circuitry.

I am using similar jetting but much bigger nozzles on the outboards.

I tuned it with a Fast wideband, and it is amazingly responsive even when wide open below 3000 rpm.