I played today with the G400. Conditions were windy and cool (11c) and very soft after a stormy week here.
Ignoring that I played awful and was hitting nothing well, I hit about half the drives well. Results were consistent with the fitting. I got a mid flight ball so it was a mid flight but more piercing than my G30. Accuracy was better, I have a fade that can be too much but this club/shaft combo was straight. In the fitting, dispersion was excellent and I was seeing this on the course. It looks like I can just aim straight with this combo instead of allowing for some left to right.
Results were positive but it's too early to say yet that it's there. I'm playing again Tuesday when hopefully I won't be so knackered from hitting a couple hundred balls the last two days and carrying my bag.