Can any performance/efficiency be gained by running a lighter primary vs. a heavy primary setup? Assume that the secondary is in tune and the correct RPM's are achieved.

Another question is if you go to a lighter primary spring, should a lighter secondary spring be used also?

To put an example out there (these are just made up combos) is there any difference between a primary setup with a 140/300 spring with 70g weights vs a 140/250 spring with 64g weights. Again I didn't say this was a valid combo, just used for theory. As long as the RPM's are correct and the secondary is performing as required can there be a performace gain by using lighter products?

Heavier = harder pull but there are so many set ups depends on what you want to do. Tune secondary for shift and primary for RPM. If you change one you may have to change another.