Yeah, it's quite impossible to call, really.
I guess the main difference to the Giro 2010 was that Arroyo was much weaker than O'Connor. The advantage was also much bigger, but it did feel somewhat inevitable that he wouldn't hold on despite the final gap not being that big in the end.
In 2006, on the other hand, Pereiro had a smaller advantage than O'Connor and not quite the previous GT results that O'Connor has (although O'Connor's results look somewhat better than they really are). And while Pereiro, like Arroyo, didn't quite make it in the end, he did beat Klöden, Sastre etc. and only lost to Landis due to yet another miscalculation/tactical error by several teams.
As
@YavorD says, there is previous for a sudden race leader to find a new level. I feel like O'Connor's gap is the perfect size for a very interesting race. Unless he just ships 3 minutes on Sunday and everything we've discussed turns irrelevant. Anything can happen, really. And I have a tiny hunch that Roglic might be a bit like 2012 Basso. Not much evidence, but I have a feeling he might not be as strong as expected.