To use a different term to say what Jprog stated, spray DA's have very low turndown ratios, I seem to remember as low as 2:1. On the other hand, tray units, and spray/tray units have higher turndown ratios, but I can't remember the ratio, allowing your BFW flow to fluctuate much more without performance loss.
I seldom encountered spray types, as the types of boilers that I had experience with had widely swinging loads.
Probably back at start up, you were running the boiler at lower loads due to some steam user not being yet started up, and the DA suffered. Once everything was started up, and the BFW flow was at rated flow, it did OK.
In a spray unit, the spray patterns through the nozzles is a function of the flow and pressure drop across the nozzles. At lower flows the spray cones are not so good, and the DA cannot work as designed.
That brings me to a problem I see with the variable pressure operation. At 20 psig, and some pressure at a given flow produced by the BFP operating on its curve, you have a given differential pressure across the nozzles. If you change nothing but the operating pressure of the DA, then you automatically have 30 psig less differential pressure, which will cause the spray cones to vary.
So, unless you are running a turbine driven or VSD electric pump, and can speed it up to match the 30 psi delta p loss, you will have to take that into consideration as well.
rmw