Well, I just fitted a 0.3mm water injection jet before my turbo with good results. I also have a 0.4mm jet after the turbo just before the plenum.
My ECU will log a fault code and cut boost if it sees over 20psi. I set my actuator to 20psi exactly and I've never had a high boost problem. After installing the jet today I went for a drive. The first time I put my foot down, the ECU light comes on and the boost is instantly cut.
What this tells me it that cooling the charge before the turbo does make the turbo more efficient. This is because the cooling effect makes the air more dense before it goes into the turbo, thus the turbo has to do less work to produce the boost required. As I left the actualtor where it was, the turbo was doing the same amount of work, so was producing more boost, more than 20psi!
This means you could run more boost without making your turbo work harder!