Yes, an actual RP competition, not just plain writing, will be exciting and challenging.
A thing I don't like, though, is planning what is going to happen beforehand in a limited number of posts. That's why I like spontaneous RPing, because anyone can take the storyline and run with it. I do that when I'm writing, too -- I plan out a basic plot, and drive towards that plot while adding in all these details and taking a few detours along the way. That's my reason for wanting unlimited posts in the RP -- so that the plot can go anywhere. If I know what's going to happen, there won't be any joy for me in writing it. I'll get bored, and my writing will be messy and half-hearted.
I know that the competition can't cater to everyone's personal needs/requests, and that unlimited posts might put a drag on the judges to read, but for me at least, it'll be funner.