Bald One is right on. The 10-20 degree superheat you referred to is for TXV systems.
Hot ambient outdoors and cool temps across coil indoors will result in low superheat with a piston due to both reduced heat load and increased mass flow rate through evap due to higher head pressure from higher outdoor ambient.
Have I mentioned that I think pistons are crap?
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.