When it comes to real-time control of parameter via midi, the original DX7 is about worthless. I you change the sound parameter values during a note, the sound will stutter. The DX7II, hovever, will handle it just fine. With the control panel I use, I can routinely adjust several parameter values at once without any problems.
The only thing you have to watch out for is sending too much data at once. If you shove a whole bunch of data to the DX7II full bore, it will lock up due to a buffer overflow, as it can't actually process the data as fast as the midi channel can shovel it in.