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It will be a while before I can prove it, but everything
I know about the process at this point, suggests that Rusty
is on the correct track in moving the injectors down close
to the housings.
The effective runner length will be altered by where you do the injection. The speed of sound is reduced by about 10% (perhaps a bit more) when you add the fuel and vaporize it. This would mean that the effective runner length is reduced by about 10% when you do the injection on the upstream end of the runner. This would shift the RPM resonance point upward by about 10%.
Who knows? Maybe this is why they mount secondary (high speed) injectors on one end of the runner and the primary injectors on the other.
As I understand it, the fuel on the walls does not move around much, but just lays there. The only problem with up-steam injection that I have heard of is throttle response. On abrupt opening, the mixture goes too lean for a moment. On abrupt closing, the mixture goes very rich for a substantial time. If you switched off the up stream injection at some mid-point throttle position, and shifted over to 100% downstream injection, I would guess that the throttle response issues would vanish.
Again, just a guess.
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