20 inches for the primaries ?
Is that to maximize h.p. when using them ?
To my way of thinking , there is a balancing act when you change anything in header design.
A longer primary will cost you peak h.p. , but typically will enhance torque.
I don't know that these will have the same effect as a merge collector , where a smaller primary can be used due to scavenging being enhanced.
Given that they have a 3" collector to finish , I don't know that they had high r.p.m. in mind when they designed them.
But I think you could go longer than 20 " and not lose much.