Hitsugaya's Bankai may be more powerful than Senbonzakura Kageyoshi, but the boy himself isn't stronger than Byakuya. He has the potential to surpass him, but as he is now? Nope.
As for his upgrade, I see no time limit and a greater area of effect. His end goal should be trying to achieve Zanka no Tachi like power. The potential to freeze the whole of S.S.
I'm glad we agree that Byakuya is stronger than Hitsugaya.
The kid only managed to imprison Harribel, when she was freed she was no worse for wear.
Byakuya barely struggled against Zommari and he finished him off fairly easily, with a regular slash no less. All of his wounds were self inflicted, one of which he had only gained because he was protecting Rukia. Toshiro on the other hand, was struggling with a non-released Harribel while he was already in Bankai. He then was only able to fight her to a standstill when he had an apt amount of moisture in the air and his petals were steady melting. Had he not used Hyouten Hyakkasou when he did, it is highly probable he would have lost that fight.
And yeah, Byakuya barely struggled, he was just cutting himself not to be controlled, fighting against opponent he shouldn't have any problems with. And yeah, he finished it with a regular slash. He didn't use Gokei at all. He didn't wound him enough for Zommari to be on a brink of death, nope.
Byakuya can't hold a candle against Ichigo, let alone Hollowfied Ichigo. The same Ichigo, that was roughly Grimmjow's equal and won by a hair's breadth. Hitsugaya took care of #3, when Byakuya wouldn't handle #6.
The issue with hitsugaya is that in the past he could not really use the full magnitude of his power, he couldn't handle it. It might be due to youth or inexperience however the matter comes down to the fact that hitsugaya was using only a fraction of his bankai's power. However IMO the scenario is not that hitsugaya will now be overwhelmingly stronger than everyone else thanks to him seemingly mastering his bankai. Rather than that the scenario will be that hitsugaya will have a bankai on par with what other captains can do with theirs so to speak. Hitsugaya's ability is flashy and has a great scale however when it comes to captains appearances are not a defining factor. Remember when gin and hitsugaya fought? That was the only instance in the manga where hitsugaya went all out with his shikai and used his dragon in conjunction with weather control. The mere collateral damage of his shikai made fodder out of kira at the time... And yet when he fought alongside the VCs against the espada there was no colateral damage. And he was in the human world. Its not like gin with his far less flashy shikai was at a disadvantage either, he actually held his ground exceptionally well against a much larger shikai so to speak. So when hitsugaya fought yami he was using a severely less powerful version of his shikai so the case can be made that if hitsugaya had been using his weather control yami would not have been in such a great situation.
So when it comes to hitsugaya's bankai the situation should be the same when matched up to other captain bankais. Hitsugaya being able to use his weather control should only put him at the level of the other captains when it comes to bankai. Before he was using a substandard bankai which wouldn't really have held up against other captains bankais. Heck, his victories when it comes to the espada only came when he went beyond the scope of his substandard bankai so to speak. Ordinarily he did not even defend all to well from luppi when they fought. Hitsugaya had the upperhand only when he took his time to use a technique which he would not be able to use casually. Against harribel it was pretty much the same scenario, he had to wait until he could actually use some measure of the true power of his bankai.
Shinigami battles are ultimately battles of reiatsu, how flashy or big a bankai is to that is irrelevant.
It's not like he had to wait.
He didn't know if he can control it in Bankai, but knowing he can not make it in time (petals gone), he had to try. He used his Bankais actual power for the first time and...
Hallibel was pretty much done. If she stayed there longer she'd be dead. When Vaizard butted in he couldn't really use it again. Or he couldn't use it all? Doubtful, but anyway, too bad.
That is pretty much my point. What we regularly see is basically just a fraction of his bankai's power which is why what we normally see is less extreme than what we usually see when captains use their own bankais. When hitsugaya uses his weather control alongside bankai or has the time to prepare he has a bankai which is every bit as extreme and powerful as that of the other captains. Otherwise what we see is substandard when compared to other captains bankais.
I guess it depends on the translation but the impression I had was that the petals being gone was a factor in hitsugaya being able to use his bankai's full abilities.
In both cases he said he didn't "want" to use it. Maybe he gets weaker with petals disappearing and he could control only that much of this technique's power. Which would mean that the technique that one-shotted #3 was at only half of it's original power. But I think that's unlikely, so I'm pretty sure he had to end it before his petals were gone so he used it.