People who go to briefing and who get cat 1 often get complacent and want to rush at the main board, getting a cat 2 or a cat 3 isnt the end of the world, and gives a lot of people a kick up the backside. Id bet that quite a few cat 2 and cat 3s outperform cat 1 at main board, like the bloke above said. Cat 4 means that you will not be able to take your application as an officer any further, and I think that these people could be weeded out by the ACAs quite easily. Youd have to be pretty aweful to get a cat 4 at briefing, tbh.
Re the int corps, one of my mates is an army officer, although not in this corps. I doubt the int corps would offer sponsorship to anyone they thought wouldnt fit in, their acquaint visit is one you can only go on once youve passed main board, and they are a very popular, but quite small corps. (They dont need to sponsor more than they need to). If you are suited to being an int corps officer, theyll offer you sponsorship. Dont listen to people saying how tough it is, they probably wish you were in their shoes!
Other arms, such as the RA, sponsor a hell of alot more because they are the largest corps in the army and therefore need sheer numbers.