There seems to be a general agreement that MBTS and other armoured vehicles do well with ~20 ... 22 hp/Mg and nothing more is needed. I had a look at Ukraine's T-84, which seems to be going up to 30 hp/Mg (source https://en.wikipedia.org/wiki/T-84). Does this make sense? If so, shouldn't we be looking at this for our MBTs as well? 

graylion said:

There seems to be a general agreement that MBTS and other armoured vehicles do well with ~20 ... 22 hp/Mg and nothing more is needed. I had a look at Ukraine's T-84, which seems to be going up to 30 hp/Mg (source https://en.wikipedia.org/wiki/T-84). Does this make sense? If so, shouldn't we be looking at this for our MBTs as well?

Actually the power to weight ratio generally concidered adequate for a modern MBT is 20 kW/t. Which is ~25 hp/t (metric)

This is the power to weight ratio the original versions of the M1, Leo 2, Leclerc etc as well as the T80U had.
The T 84 with its 883 kW engine and 48 t weight does not reach this high. Its in the 18 kW/t range. Which means its about the same level as the Leopard 2 A6 with its 62 t weight. So its not really as good as you make it look. Its a solid average.

The lower power to weight ratio of the upgraded Western legacy designs is a result of a shift of focus and the fact that the 40+ old designs are mostly getting upgraded protection. Which is fairly easy and cheap. Just slap more armor on... simplified. Improving the drivetrain is much more complicated. The 3rd gen MBTs have never been intended to stay in service that long. While some improvements and weight increases had been planned from the start the level we see now has not. To get back to the power to weigh ratios as originally designed a Leopard 2A6 would need a 1600 kW or 2200 hp engine... which means it also needs a new transmission, final drive, drive sprocket, tracks, track tensioning, suspension, shock absorber... in short the you need a new or at least extensively redesigned hull. At this point it becomes more sensible to design a new tank alltogether.

schnuersi said:

The T 84 with its 883 kW engine and 48 t weight does not reach this high

Later version has a 1500 hp angine, bringing it to 30hp/t or 22kW/Mg. OK, point taken ;)