Any tank can be upgraded to operate in a low oxygen environment by adding a turbocharger. I assume tibet is fairly mountainous terrain with poor infrastructure which makes it more difficult to operate large MBT like the T-90 or ZTZ99. Hence why India is also scrambling to buy light tanks as well.
No, certainly not enough.
How do you think 15 achieved a power reduction of only x% while on altitudes of 5000? Obviously not just a turbocharger, the compression of turbochargers also lower when you've got lower environment pressures. 75% is a rough number applicable to virtually all tanks including the T-90 and 99A, which has a turbocharger, when on plateaus. 15 makes use of a second stage conpressor and inlet air heating to achieve the higher numbers. www
Also, your claim of any tanks can be upgraded isn't true at all since you can also say we can add anything to a tank to make it better in any extent. Given the time and the technology, you can modify a T-54 to make it as good as Armata. www