Since I brought up the converter, those numbers look like something in the 10% range of slippage up top to me. Actually not too bad in my opinion.
That 5600 is about as high as that combo will make maximum power, so I agree with the others that timing & jetting can pick things up. Good tuning can build more torque thruout the rpm range, and that alone will get you some more mph & as a result, more rpm upstairs as well.
