When you're talking performance, usually that means driving as fast as the machine will go and still do a good job. There are several areas of the machine that can be limiting factors. (Sickle bar, feeder house, rotor, sieves, clean grain elevator, engine HP, etc) Depending on which area you're reaching capacity at will determine the performance difference between two machines.
The 88 series machines run almost completely on belts, while the x010s used a lot of hydraulics. I don't know any official numbers, but I believe the belts are going to be able to transfer power with less energy loss than hydraulics. Thus if you're reaching the engine HP limit before anything else, the 2588 might about be able to hang with a 7010.