I actually tend to leak more when wearing a onesie then when not, but I do like onesies because the hide the diaper better and keep it more compressed (less sag) hence the leaking more, it just means more routine changes though.
Depends on what it's doing. If it holds the leg gathers up against your body more, that helps prevent leaks. If it presses up on the middle of your diaper and closes the "channel" between your legs where your diaper drains to the back padding, (or turns the "V" fold into more of a "W") it'll leak more. So how it fits you really determines whether it helps or hurts.
I second (third, fourth, fifth, ..) that a onesie or compression cover helps in covering up or reducing the appearance of a diaper but does not reduce leaking. I wear onesies for one major reason - to prevent anyone from seeing what I don't want them to see. I do get wicking at times when I push past the point of needing a change out.
Also, wearing an over tight (body suite like) reduces the pouch (has been mentioned) or droopiness of the diaper is a catch basin for all sorts of stuff. However, it causes more leaking.
For me, the onesie reduces the effectiveness to the benefit of not having a noticeable (psychological or actual) rear end. Even after years of having to wear protection, I'm still aware and self loathing at times of having to be diapered.
So will they keep your shirt from rising above your apron? And do i still need to use underwear over diaper. At amzon which one is best. Is there one that you don't. Need to take off your clothes in order to put on?