There
*may* be a difference in the winding of the motors that would affect the speed (RPM).
So, if you put 48V into the 36V motor it
*might* spin at a higher speed than putting 48V into the 48V motor.
As an example, cell_man offers a direct drive motor with 3-4 different winding options:
http://em3ev.com/store/index.php?route=product/product&path=40&product_id=111
The listed RPM values are with a 36V input (I think). Each motor would spin a percentage faster with a 48V input.
The winding will determine the speed at a certain voltage. You can (almost) always go to a higher voltage for a faster speed (or a lower voltage for a lower speed).
The wire thickness will determine maximum current which will determine the torque. You usually don't want to increase the current too much (this is how you burn out a motor... the wiring heats up and the insulation melts causing a short circuit).
Wattage is just voltage times current and doesn't mean much by itself (other than a comparison to other motors that are fed similar voltages). For a given motor, you can increase the voltage while keeping the current the same, and the wattage will increase. The heat generated is mostly due to the current NOT the wattage.
So, given the same winding and the same wire thickness a 36V 800W motor is probably the exact same as a 48V 1000W motor. If they have a different winding and/or wire thickness they are different, but either one could be made to go faster by increasing the voltage.
Maybe the seller can provide more information?