Well, that's mostly right. The stronger the spring (thicker) the higher the RPM's will have to be before the clutch engages. Going to thicker springs also gives you more punch off the line, as the engine will be closer to the usable power band when the clutch hooks up. It does not have to work harder however - the amount of load on the engine hasn't changed. The only thing that changed is the point where the clutch engages the bell.