Super Slam is really not great. It's OK on easy and normal and almost useless on hard. Let's see, each point in CON at lvl 25 gives you: 24 health (30 with juggernaut) on Hard, 51 (64) on Normal and 102 (128) on Easy. That means that at lvlv 25 each point in CON gives you: 6 dmg on Hard, 13 dmg on Normal, 26 dmg on Easy. Damage that you can apply only once or twice per combat. Each point in STR however gives you 7-8% damage increase from boosting your skill and 10% damage boost from having STR above 8. I'm not sure if they are additive or multiplicative, but that damage applies to every attack and also STR increases your accuracy with a weapon, so it's clearly much, much better deal.
So i don't think that Super Slam worth it in 90% of the cases even if it multiplies on the crit. For sledgehammer users it's much more viable to put their point in STR rather than CON (that's being said, having 7 CON is recommended). A nice bonus is that if you have 15-16 STR then you can put adrenaline and rathound barbeque on top of that and use Balor hammer for about 3 rounds in combat, which often is enough to eliminate your enemies, given that with that weapon it's usually 1 hit = 1 kill.