SLAs are more of a financial construct than anything else. Once the payback cost of missing an SLA is built into the contract then it just becomes a conversation about money. I've been at plenty of shops that obviously tried to hit the SLA but if it was missed it just became a financial issue which helped smooth over what otherwise might have been a trust buster.

I would never ever think of an SLA as anything more than a financial commitment - if you think more of it you'll eventually be in a world of hurt.