I had a similar issue... but it sounds like you've worked down the same path I did.
What fixed mine was a combo of the plunger not hitting the ball square and the machine wasn't level side to side. It was sloped correctly... but slightly uneven left to right. Seemed like the ball slowed down after coming out of the shooter lane because it bounced around a couple times before it got to the gate area.
I guess if it is slow right out of the shooter lane it must be something with the coil.