Our 30LPM Pump is running through a 21 litre burner so same analogy. But we don't need to run heat at 30 LPM. We'll only use heat for gum so knock the revs down.
Do you need heat at 21 litres per min?
I think Trevor Perry's burner is 15 Litre and on a 21 LPM pump and there's good heat there, I've seen it when he worked with me.
First thing I would check mate is if your flow switch is ok for 21LPM. More than likely it will be rated at 30LPM. Maybe they're all rate to that, not sure, but the one's I've always been through are rated at 30LPM, 250bar.