Heh guys thought I would share my story. Bought a new 3903 about a month ago. Being a RC helicopter pilot, I had a high power charging setup so I bought a couple Gens Ace 5000mah 40c lipos. That woke the truck up quite a bit however I found myself wanting more power.
So I bought the Hobbywing SC-8 WP and a Hobbywing 2000kv brushless motor. I am running a 21 tooth pinion with the stock 68 tooth spur gear. All I can say is WOW. Tons of power down low and good top end speed. I would highly recommend this power upgrade. Total cost was around 150.