TL;DR
Just check out Noah's posts.
China already kicked our ass in a war once and that was back when they had primitive technology.
In the Korean war after the U.S. destroyed North Korea's army, General Macarthur stupidly threatened to invade China. China sent troops across the border and drove the U.S. all the way back to the 38th parallel. China completely destroyed a U.S. army division and forced the Marines (who claim they never retreat) to retreat.
The U.S. had overwhelming firepower, far superior technology, and total control of the air. Despite the misleading belief that the "yellow hordes" outnumbered us, manpower was equal. The Chinese soldiers were better, tougher, and more courageous, and they used better strategy and tactics.
Now that China is catching up to us in technology, what would happen?
While China did indeed succeed in pushing the UN out of North Korea, and their demonstration of tactics is impressive, the military forces were not so even as you seem to believe. The number of Chinese troops far outnumbered the allied forces in North Korea. In all engagements of the Korean war with the Chinese force, the Chinese took a roughly equal, and in some cases far greater number of casualties than the allies, yet still held superior troop numbers, even with understrength units.
Many of the Chinese troops that fought in the Korean intervention, especially the officers, were coming out of the Chinese civil war, which gave them twenty years of battle experience. Those officers and their troops under their command were still full of zeal from their revolution. While this may be a large factor for their victory in North korea, those factors don't exist today.
Several other contributors to Chinese victory that would likely not exist:
-The North Korean and US army troops at Unsan were taken by surprise and completely outnumbered by the Chinese invasion force. The technology of today and the nature of the conflict in Taiwan (that is, we would be expecting to fight them) would make such surprise difficult to create.
-The wars of the future will be fought with professional soldiers, that is, you can't just toss someone a bayonet and grenade and send them to the front lines expecting results. In the battle of the Ch'ongch'on River, the US troops lacked a lot of discipline, many troops foregoing equipment because they were so certain of victory. In the same way, the Chinese couldn't expect to field a massive army and expect ot come out on top with the help of superior numbers, which are rendered much less effective with current artillery, missiles, and other technological advancements, such as personal machine guns.
-Chinese weapons during the Korean intervention were more advanced than you may think. They captured weapons from the Nationalist army in the civil war, many of which were supplied by the US. They still used these by the time of the Korean war.
-Forests. While the US had air superiority, that doesn't mean much when you can't tell where the enemy is at. The Chinese forces also made sure to move at night. Again, advances in technology make this tactic more difficult with things like thermal vision, even imperfect. Bombs are also more precise now. Today I can mark a target and have a bomb dropped
right there, as opposed to the WW2 method: Drop the bomb and hope it hits something. It was only at the battle of Chosin Reservoir that the air force even got to see real use during the fighting against the Chinese in North Korea before the regrouping behind the 38th parallel.
A few other notes:
-Chinese victories were only decisive in North Korea. After they pushed the allies back tot he 38th parallel, the Chinese attempted three times to continue the invasion but were stopped due to the logistics for their army being insufficient and the revitalized allied troops. This led to the current stalemate.
-The Chinese logistics were horrible, and a contributor to the Chinese Pyrrhic victory. Mao sent some of his elite units to the front lines at the Chosin Reservoir with no advance warning, winter gear or other vital equipment. More of the Chinese dead at the Reservoir were due to the cold and lack of supplies than actual battle. Their lack of naval power also enabled MacArthur to successfully evacuate huge numbers of troops and supplies at Hungnam.
-On the matter of Marines. While Marines may not run away, they still obey orders to retreat. I'm not sure the slogan you're referring to even existed during this time period.
All of this is not to say China couldn't win in a fight for Taiwan. But if they did, it would not be for the reasons you cite.
Although we wouldn't have the Turks with us this time...Those Turks I tell ya...their entire brigade is fragmented, a huge portion of it dead, they're using rocks to fight off the Chinese and their General asks "Withdraw? Why withdraw? We are killing lots of them."