The difference is that the 790 has more power at the topend and the 865 has more torque at the bottom end. This means the 865 will be quicker starting off and the 790 has a faster top speed.
BUT the reason for the difference in power is the valve timing is very mild on the 865, especially on the exhaust cam. If you start with an 865, open up the breathing at both ends of the engine, and install better cams, it would be a different bike.
