We can debate the validity of EMG all day long. Let's not. Mecanically palms neutral rotates the real delt to a worse line of pull. I don't care that they are firing more per se. I care that they are in line with the line of resistance. EMG won't determine taht since it only looks at gross firing levels.
And we're not talking about RC stability. packed shoulders may improve that. But it also will decrease the amount of horizontal humeral ABduction.