An advice that works for me on hitting penetrating forehand with an horizontal swing path is to avoid the raquet head drops down the ball.
It was easy to me to change the swing path ..it was less easy to change the habit to drop down the raquet head relaxing the wrist/forearm.
The sum of horizontal path + raquet head drop down is perfect if you want to make a hole on opponent back fence.
To hit flat, swing horizontal and move the raquet head back instead to leave it drops down as the forward swing starts.
Wilson BLX six-one TEAM + 8gr lead - Gosen Eggpower 23/22 Kg