Question about pepping up my electric go kart.

My son and I recently modified his Serengettin Solar wing go kart from a 24v 350w set up to a 1000w 36v system. This included a 1000w motor and controller. I am using 3 12v 7.5Ah AGM batteries. We though it would be considerably faster however it is not much faster on the top end however on the bottom end has much more power. Does any one know any ways I can get more top end? Were at about 12 mph though we would be closer to at least 20. I do have an additional 12v battery as well as another 24 volt. Any advise would be helpful. I can be reached by email at jaegers3 [at] yahoo.com.