Nesta,
One thing is for sure, gmatprep is most accurate indicator among all.
As for me and gmatprep score, my quant was always decent, but max I scored in gmat prep was 45 and least was 42. My concentration was always on verbal as I used to do a lot mistakes in that and thats what I did in gmat as well.
As for test score, I scored 680 (Q- 49, V -34) and my gmat prep score too hovered arnd that. Between 650 - 690( depending if got some familiar q that I had already done in previous test). My major issue was (is) that I never created the actual test scenario at home during practise, and sometime I used to lose focus in quant as it used to be too easy!(Read: A lot silly mistakes in gmat prep test). But I was pretty sure that of doing well in real thing and I guess thats what happpened, I was pretty focused during the real test so I guess that explains the difference.
But apart from gmat prep, I did a LOT stuff from gmat club mostly quant, and I reckon to do well in quant in gmat, one needs to go up a level and
gmatclub tests and questions are pretty good in that regard.
Regarding stress, well i ain't qualified enough to comment, but if I talk about myself, I was confident of doing well in quant, and may be I had a bit of good fortune as well! Looking back I reckon I was definately stressed for first two questions but after that I was not or I can't remember whether I was!
I am going to take gmat again in couple of weeks time so, lets see whether this attitude works again or not!
Let me know if I can be of anymore help ..
Cheers,