How reliable is reliable enough, on a gun you're needing to avoid death at the hands of another? Pretty darned reliable, I'd say.
We each have different levels of comfort for how far we're willing to extend trust in a defensive gun. Myself, I'm pretty comfortable that if 1000rds of all sorts have shown themselves to be highly reliable through a given gun, then it's highly likely that gun's going to perform when the chips are down ... 'cause I've already thrown everything at it I can think of, and it's performed as near to flawlessly as I can hope for. Perfect would be better, but I've only seen that on a couple occasions, and only after lengthy break-in on one of those. So, of dozens of defensive pistols, only one has performed this way, with only one being a close second.
That's why I much prefer testing it. I'm a bit of a "show me the money" sort, when it comes to defensive firearm performance. Better than blind faith, IMO. It's a bit like the old Porsche motto: "There is no substitute."