The masturbation issue is very easy to figure out.
As westla and others have said (over, and over, and over again) ejaculating won’t change your gain potential at all. It’s about timing.
If you ejaculate during your PE session, that usually ends the session. You lose further interest in the workout and that robs you of some good PE time.
If you ejaculate right after your workout, your penis will retract and you will lose some of your post-PE engorgement. If you are a Wrapper (TheraP or ACE) this can be a disadvantage, however you will get a lot of your fullness back when your refractory period has ended.