Can this power burner be downfired to save fuel?

Very unlikely that you can downfire that boiler to come even close to your installed radiation. You have an extreme oversize situation there.

With 187 EDR 240x187=44,880 btu load x 1.20 (80%) efficiency
=53856x pik up factor x 1.33=71628btu input.

according to the burner name plate you can go down to 50,00 btu/hr

BUT you limiting factor will be the boiler stack temperature you need to prevent flue gas condensation. So keep your stack to about 350 check gas pressure and adjust fire with combustion test equipment. You need 75,000 minimum input if the stack temp is high enough. Lowerring the firing rate will smooth things out and save fuel as long as you can get decent combustion. Gas utilities put thousands of those in. Not a big fan of that burner but they work

I installed over 2500 of those Midco E-20 burners. Properly set up and tested they are fantastic. They are also very forgiving if you have to derate to increase efficiency with out loosing stack temp and good combustion analysis. It takes patience and also knowing all the tricks such as baffling, neutral pressure point adjusters etc. This is all covered in my manual on conversion burners.