No it isn't. Why should someone have a "straight back" and then, when they reach some magical threshold (scratch?), switch to having a rounded back? Having a straight back, chin up, butt out is going to INHIBIT his ability express the talent he has.
Already covered this. Click and read the URL above, Pablo.
So are you saying that rounding the back is an old man problem?
I disagree that his back is "very rounded." It's a bit more than we typically see but it's well within the range. In fact, I wouldn't mind seeing him let his chin drop a little bit. It looks like he's still trying to keep it up high.
I suppose if the OP had a overly strong grip, I could point to Lee Trevino's VERY strong grip and state 'nah, don't change your grip. Lee hits it well that way........' Just because someone CAN play exceptionally well with an unorthodox setup doesn't mean the average player can.
Please do some reading on this. I'm okay with a wide variety of setup positions, from the super-slumpy to the fairly straight, but telling someone to go towards straight-back-chin-up-butt-out "just because"* is silly and counter-productive. If you want to discuss posture in general, the thread linked previously is the place to do it, not in this guy's My Swing thread.
* (Click to show)
I say "just because" because you've really not offered any reason why he should change his setup. What problems is it causing and what problems will it fix? You've called his setup "extreme" and "not ideal" without defining the extremes, the neutral, or the ideal. It seems like you read in an article somewhere that you should have a straight back, and chin up, and butt out (and I know you haven't said all of those things specifically, they just tend to carpool together if you get my drift) and so that's what you believe to be true. How about being able to look at the ball out of the middle of your eyes? How about the fact that the shoulders are rounded in at impact and not pulled back, so why would you set up with them pulled back, as that changes your radius to the golf ball? There are a lot more reasons to round your back slightly, have your chin down, and your hips a little bit further beneath you instead of sticking out backwards to thrust out your rear end. Read the thread, contribute there.