Not sure it matters where the restriction, if needed, goes. Chips have to pass it before or after the blower. If using a separator put it on the discharge since only dust comes through, and since this is for your lathe, only small particles/dust, put it on the discharge.
As far as the difference between a restriction on the inlet vs discharge side of the blower, theoretically there would be some difference in amps, not sure of the magnitude. Assuming the airflow is the same, the restriction on the discharge side would create higher pressure resulting in higher density air which requires more power. It gets a bit tricky though - I said for the same airflow. With higher density air the blower will put more cfm through the system, and draw more amps. Its the change in density or specific volume, not just air volume. The only way to do the comparison is to measure inlet flow ahead of the restriction, and adj the restriction size, inlet or discharge, to achieve the same inlet cfm.
A DC will draw more amps without a filter. this is not the same as described above, because the cfm's are not equal. DC impellers work by imparting momentum to the air. They have a lot of "slip" as restriction builds, unlike a shop vac blower that is designed for higher differential pressure. As a filter clogs, pressure builds and cfm drops, and more air is "slipping" vs being pushed out. and the impeller is actually doing less work. Remove the filter and slip is dramatically reduced, and the impeller does more work pushing the air out.
As a filter clogs less mass is moved, cfm drops. The reduction in cfm