Is it possible to walk into the exam with a good idea of how you'll do?
To be honest only to a [small] certain extent. Even walking out it's difficult to know, but before it has started there will always be nerves that there will be a bad topic or you'll have writers' block. I didn't really want to talk to anyone beforehand and felt quite weird.
For me, the exam actually went as some people above described, in fact I distinctly remember looking up after my 2nd essay was done, with a good hour to go, and thinking, "This will feel awesome at the end". I imagine I definitely smiled. On the whole it just felt good, and I finished with a little time to go and actually didn't change anything.
That being said, I most certainly didn't expect a 50 (though didn't see any specific reason why not), but probably anything above a 45 or thereabouts.
And I most definitely made at least one textual error (in Context though), referring to one woman by her daughter's name (Omagh's Patsy as Kathy for those interested).
I did screw up later that day in Spesh, but perhaps that is for another thread.