Sorry, but you're gonna need far more than that to outgun China, who will have all their military assets to choose from. If South Korea get involved in any conflict, then North Korea won't sit idly by. If USA, Japan, South Korea, Philipines and Australia participate in an assault on China, whether actively or just by hosting US forces, then Russia will take a great interest in proceedings, considering they just signed that colossal gas deal with China and they won't want US regaining more power in the region. Any war involving multi-allied forces from the US may precipitate into an all out World War.
My guess is South Korea would almost certainly refuse participation in any conflict in the name of protecting Japanese or Philipines sovereignty. They're right on the frontline and don't really have any beef with China and have good historical and current ties. Australia rely more on China, economically, than they do with the US, and when push come to shove may expel US forces, if they were forced to pick sides in a war with China. US hegemony in Asia is teetering on the edge and will weaken as time goes by and China becomes more influential. The US are not as mighty as you believe.
You may have all the assets to choose from in Asia, we have 7th Fleet itself that is equivalent alone that can handle the Chinese navy. With U.S. allies in the region and perhaps European allies its going to be a lot harder for Chinese to fight if they think they can take on just the smaller Asian nations. With this in mind, it would deter China by reminding them what could lead to WW3. Don't think the U.S. is not afraid of China because otherwise China would have already taken all of SCS.
I was stating in case of an all-out full-spectrum war but agree that is highly unlikely as China and USA have too much to lose. Like I said, 300 subs is excessive but it is achievable if China were ever pushed to do so.
Compared to USN's ASW capability, I agree China is still behind but it's not non-existent and if forced to churn out many new ASW helis and subs, China can and would. Also, remember in litoral waters of SCS and ECS, smaller and quieter diesel-electrics which China have many of are very potent weapons against US nuke subs and surface combatants. The US's surface warships will have to operate far out to avoid being taken out by China's massive arsenal land-based missiles and this will affect the effectiveness of their ASW capability and US subs operating in littoral waters are at high risk to being sunk by the more numerous and stealthy diesel-electric subs, China will field. If US wants to fight China over Japan, then they will have to risk losing some very big and expensive ships.
The question is China willing to lose one of their only carrier alone with every ship that have in their fleet? With stealth cruise missiles and bombers as well as submarines and mines, there be nothing left of the Chinese navy.