A CBG in the vicinity of Taiwan when the PRC began it's amphibious assault would make a GREAT DEAL of difference. However, should it ever come to pass, there wouldn't be one CBG, but a minimum of two (IMO).
As for subs, the best use for SSN's in this scenario would be to blocakde Chinese ports and naval bases, and mine the approaches to them. The naval aspect of this kind of scenario is very limited in scale, actually, and consists merely of keeping the PRC from launching an amphibious assault upon Taiwan, maintaining air supremacy over Taiwan and Japan, and interdicting/disrupting PRC trade. After that, it reverts to politics since there is certain to be no actual ground combat. Neither country could successfully invade or maintain control of the other's homeland.
And this is where the important part comes into play: despite the actual numbers of combatants or ships engaged, the war will be won or lost politically on either countries home front. I feel the Chinese have a lot more to lose here than they would in face-to-face combat. There's your ultimate weapons: 1.3 billion Red Chinese pissed off at their own government.