I´ve tested your approach at home with a mirror (the hopper out first) I find this very hard to do actually!! When I play I come out the other way..
I find that coming out your way, you have to strech your body a lot more. Comming out with your hopper first is like a tilt in the hips, whereas the other way you use your hips to move the upper part of your body sideways... So plain physically if find this harder to do...
Like someone else pointed out, keeping your elbow pointing at your stomack, will produce a smaller target. By coming out hopper first you show yourself before your barrel clears the barricade and you can fire, giving your opponent more chance to react. When you go back in your barricade your hopper will be in last.
On sub air you get another advantage from keeping your hopper in. If you use a longer barrel, say 14" You can set up the shot before coming out. Stand in your barricade with your hopper tucked in and use your barrel to push the sub air, this way you only show your barrel, half your mask and a small part of your shoulder. It is also quick to get back in, you just stop pushing the barricade....
Try it out, it takes lot of practice to play tight!!