I think I actually calculated this at one point, it was about 50 kilojoules iirc, and was consistent with a statement from a freshly augmented John where he felt confident about surviving a 30 meter fall which was 41 kilojoules.
In regards to your calc, you said this,
“ We have a sketch of the exoskeleton the guard was wearing. I don't think I can figure out its mass. However, the Mk lll MJOLNIR exoskeleton was stated to be bulkier and heavier than te newer versions of Mjolnir, witch weight about 450 Kg when occupied „
And decided to use 450 kg as the weight of the Mark 1 suits with occupants used by the trainers against John and Blue Team.
But I have a question regarding something I read,
“ In its final phase, the armor weighs half a short ton (453.6 kilograms) „
~ Halo: The Fall of Reach (2010 edition); page 2 & page 16 (Regarding the Mark IV MJOLNIR)
Does the term final phase mean "the complete armor without a occupant" or "the complete armor with its occupant"?
Because if the 453.6 kg figure refers to just the armor by itself we could add the average weight of an adult human bringing the total weight up to 518.6 kg.
The weight of an adult human, or a large dog: 50 kg - 80 kg (Avg. 65 kg)
~ Lifting Strength Page
Using your calc with the 518.6 kg value results in a slight upgrade
High End: 0.25 seconds, 32 m/s, 518.6 kg
KE = 265,472 J Wall Level
LS = 66,380.8 N = 6.768957799 metric ton-force = Class 10
Mid-End: 0.5 seconds, 16 m/s, 518.6 kg
KE = 66,380.8 J Wall Level
LS = 16,595.2 N = 1.6922394498 metric ton-force = Class 5
Low-End: 1 second, 8 m/s, 518.6 kg
KE = 16,595.2 J = Wall Level
LS = 4,148.8 N = 0.4230598624 metric ton-force = Peak Human