Southern 630 has Southern gear.
Variable lead is like having advance in the distributor of your car. The faster you go, the more advance you get the better the engine runs at higher RPM. Same for a steam locomotive. The faster you go, the higher you hook it up, the earlier the steam is admitted to the cylinder before it hit dead center, the more cylinder pressure you get and more power is created. As in an automobile, too much of a good thing will tear it up and hurt performance. Constant lead is just that, a compromise. There were some variable lead set ups for Walschert involving a shifting link where the valve stem ties into the combining lever and radius rod. D&RGW 1700's had this. I don't know how successful it was. I seem to think it was removed later on.