I'm sure it does help to practice under pressure, but when you're in the 30-40 second range, I'm not sure being nervous hurts all that much. I was so nervous at the US Open doing the 3x3x3 that I was about to drop the cube from the uncontrollable shaking, and yet I got times that for me were actually VERY good. My average in the B final was 39.80, and my personal best at home prior to that for an average of 10 was 37.84; my typical averages were still over 40 seconds. I think the adrenalin boosted my speed enough to compensate for the shaking and the mental mistakes (and there were quite a few mental mistakes).
On the other hand, when you're as good as Harris Chan, I suspect being nervous makes a BIG difference. (But not enough to keep him from winning.)
So I guess my point of posting this is that, if you find yourself very nervous, don't worry about it - if you focus, you can still do well.
Another comment: I think the reason I was so nervous was because it was the "stars" of cubing doing the judging. Dan Dzoan was my first judge, and as he stood there watching, I couldn't help imagining what he must have thought of my pathetic performance. But he was actually very nice, and after my first attempt he made some comments to try to calm me down so I would do better. He was very helpful, and I really appreciated that.
And one more comment - it's looking pretty likely that you'll beat me at this point. I'm still averaging around 35 seconds.