I have several thoughts on this:
Firstly, one thing I really liked about the challenges in UG1, was that when one challenge started, the next one was announced and I knew that I had one week to create the car, so I could play with ideas, and really make the best car I could (ok, don't go looking back through my UG1 entries LOL) but you know what I mean. Now, you *have* to be on the site as soon as possible after the competition is announced, go off, build your car as quickly as possible, and get it in before the competition fills up. For this reason, I think there should be a limit of between 150 - 200 slots and see how that goes.
With having to vote for that many cars, it doesn't really take that long, especially if, as many people mentioned, you spread it out over the week. I really think it would be good if the voting was like for the tuning challenge in UG1 - after you voted on one car you got a next button which would take you to a random car you hadn't voted on - and you could come back later. Currently the cars are listed randomly, but if you come back you have no way of knowing which ones you have already voted on.
I do also think it would be good to have the new challenges announced several days before you are able to enter them, this would ensure that people at least had a bit of time to work on cars, knowing that they didn't have to rush too much.
Especially, I think that most of us would rather spend an extra half an hour voting on extra entries, than rushing to build a car only to miss out on entering at all.
I am so much less interested in the hot hatch comp because I missed out on entering
That's just my thoughts on the matter anyway.
Regards
OgO