It's definitely boiler input temp that determines efficiency, but if the delta-T on the radiation is too small you can't make it any lower by adjusting only the boiler flow. If the radiation is already returning 125F water with 150F output you can leave the radiation loop flows alone and concentrate on what it takes to optimize it on the boiler loop.

(This is the problem of design-by-hackery rather than doing all of the math up front, eh? :-) )