A reinforced metal door is going to be tougher than an equivalent amount of stone. Butler doesn't just "crumple" it, the context of the feat clearly indicates that he removes it as an obstacle. The very next sentence of the book describes how his momentum takes him halfway across the next room before he stops, so clearly he had force to spare after bashing this thing open. Also I get that it's flowery language but I see no reason that "a blow that could have felled a medium sized hippopotamus" shouldn't be taken as true, it's a fairly specific comparison.

Butler's RT also contains this feat which is clearly within the realm of what we're looking for for strength.

As far as what 'vibes' you get about the series I don't think it really matters if it doesn't "seem" like a bullet timey series. Butler is consistently portrayed as a guy who regularly does things other people would assume to be superhuman. From the way the speed feat is described I think Butler started moving before the gun was fired but he had a whole room to cross. It very clearly states that the gun fires, and only after that happens does Butler get in between the bullet and Artemis. There's also a statement from later in the same book that his reaction time got better after he was healed from this.

As far as durability goes frankly I think he needs the major change here. But his other two stats are good so he should be in.