I added a 6" Jackplate to my Z6 and the results were not quicker on the plane, but it planed out faster if that makes sense.
It rides a little better at WOT as the boat now sits further back on the pad by 6" when running.
I gained no more speed, nor did I lose any.
It looks cool.
I was able to raise my engine a little as well.
I did however have to add a holeshot plate to reduce the amount of backwash when pulling up. Because there is a 6" gap now between the transom and leg, the water forces up and into the well, the holeshot plate all but stopped this.