Very interesting Nigel!
If I remember right, the Lamb 5.7" motors used in ninjas develop approx 7.5" Hg of lift and move roughly 109 cfm
There's something I can't get my head around about this mod - I can't help thinking it's a little lop-sided set up like this. You have 15" Hg pulling from the 2 in-series motors inside the machine, and 7.5" through the booster unit.
Theoretically, on paper, you've altered your setup from moving 109cfm up to 218cfm. I can't decide whether your vacuum lift would remain at 15" or if it would somehow end up half way between the two, i.e. at 11.25"
What I'm thinking, is the stronger pair of motors trying to steal air from the weaker motor set up all on its lonesome back there?
I certainly see where you're coming from with the mod though (I think Stu Clark up in the north east has done this to his ninja too), it's a much better way of using what's there. 3 vac motors in a line seems pretty pointless to me, all that suck but not much movement (fnar fnar :p)