To Inst:
Basically there are advantages and disadvantages of heavier tank (with bigger guns) in all aspect - technologically, technically, logistically, etc. As far as I can see it (which is quite limited I admit, my knowledge in tank warfare is not as extensive as many of the posters here.), the advantage of having such a tank cannot be oversee. It do make a difference in battlefield. However as to even suggest that the development of such a tank would cause an arm race is pure and simply impossible.
If an arm race is going on... development of this tank is not the cause of it. China had many ongoing project such as aircraft carriers, nuclear submarines, conventional submarines, J-XX, J-14, DF-41 (if it is still ongoing), DF-31A (manufacturing of more), etc, all these have greater potential of causing alarms to her neighbour and causing a rapid arm race. So far we are not seeing much proof on that. If these weapons could actually get pass without much panicking, except for some tongue lashing and knitting of eyebrows, I couldn't imagine what one tank would raise so much concern.
Your assessment of NK is absolutely wrong and the way China would need to kill NK by traditional use of tanks and armoured units are also way out. Of course in the end, China would need ground force, but I can predict that that would not be the case right in the beginning. Like the US and NATO, China nowaday could use precision weapons, cruise missiles and stuff like that to put down the defences of NK, then they would send in their air force and navy to bomb the hell out of korea, and finally they would go in with their ground force. Plus... actually if you look at the position of NK, if she didn't receive any help from SK when China attack, China do not really need too much military to destroy that nation. NK is a VERY POOR nation, feeding her population is already a problem... much less arm her men with enough ammunition and weapons.
All China had to do is to lock NK from both land and sea... create a blockage and within months, you will see the nation collapsing. NK's survival nowaday is mainly from aides given by China, Russia and UN.
As to your discussing hypotheticals... well, there must be realistic truth in each hypothetical (sorry to say that). And as pointed out by what I have written in my posts, I give explanations into why some of what you have stated would not happen.
Finally a war between great powers like China and Russia, I can predict that tanks are not what will turn the tide of the war... they are good weapons, but never a trump card.
As to your scenario on US... well, by now, US could strike at any nation they want all across the globe, their transport system, logistic and stuff are too great to ignore.
Bottomline:
China is already surrounded by potential enemies, but what you have pointed out, simply cannot happen. China still need to develope the tank and many other weapon platforms, but unless it was something out of this world, secrecy is something that China would not go for right now... afterall China is also a big arm dealing nations. Who knows, China might even want to sell their 140mm calibre MBT to a third nation.