Reinforced Armour (or any armour that says "lose fast") obviously removes fast from your tank and is irrelevant as a downside for something without that ability, however, can you still use a SuperCharger to gain fast if equipped with heavier armour?

Coming from a card game background we would traditionally apply the last effect used as overwriting everything else so I'm inclined towards yes, however, this is obviously not a ccg :)

Hithero is spot on.

Oh to have an IS-2 with concrete armour charging right up the table at 3.

Mind you... Is-2, Sandbag armour, supercharger .... mmmmm get in there and start mixing it

Yes while you have a card face up that makes you lose the Fast keyword then you cannot gain Fast via any means.

If you had something like Concrete Armour that must start the game face up, and Supercharger, once the upgrade is discarded to cancel the critical hit you can once again gain Fast since the concrete armour has been discarded.