Well, why bother with armour at all?
If you can somersault in full armour, without it he must be able to double-jump...

Edit : Hang on... What "plate armours" are we thinking about here?

Is it this: [img=http://img3.wikia.nocookie.net/__cb20120114230313/assassinscreed/images/e/ea/Guard-brute-ACB.png]

Or this? : [img=http://img1.wikia.nocookie.net/__cb20120115004338/assassinscreed/images/9/92/Guard-papal-ACB.png]