Look at a video of a Sampras running forehand..he use an over the head finish a la Nadal using an Eastern Forehand grip. To generate "serious" topspin with an Eastern forehand grip, you need to have an exaggerated windsheild wiper motion.
Generating topspin comes more naturally as you move towards Western grips. Driving the ball flat is easier with an Eastern. You can do the inverse with either grip, but it's a question of what comes easier.
3.5 player. Equipment: Prince Original Graphite OS, Tourna Big Hitter Blue