If there is some sort of restriction at the end of the hose, perhaps there is a point to be made here, but when the restriction is at the manifold due to either orifices or a variable orifice manifold with adequate sized hose after the orifice, hose length is not going to be a factor beyond the wives tale degree.  The vertical dam manifold works a little differently then the varible orfice design but achieves a similar result.   

I really doesn't matter how much hose you run the fertilizer through after it has been metered.  The pressure drop at the flow rates being put through the hose is insignificant unless you are using ridiculously small hose on very wide spaced knives.  8 gpm total split up between 27 3/8 hoses does not amount to much flow even if you claim that it is 100% vapor, a contention which I do not accept.  At the temperatures where corn is fertilized (excepting sidedress) I think you will have a hard time finding very much vapor at all.  When everything is surrounded by a ball of ice there's not much vaporization going on.