The pump needs to run fast enough to ensure that the water does not boil as it passes through the heat exchanger. If the firing rate drops, you can slow the water down, since there's less heat to absorb. If the things are all lined up properly, the outlet temp might remain the same. You'd notice a difference in the return water temps, but that may not be an issue other than efficiency (which should increase as the delta-T increases). If this is the same pump that circulates water in the radiators, the delta-T from the outlet to inlet will increase as you slow the flow since there will be more time for the heat to be extracted in the loop. If I'm understanding things, the only thing being minimized is the change in temp (delta-T) of the outlet of the heat exchanger, not the system's delta-T. THen again, I may not be understanding things properly!