It's a good idea in theory, but it requires a handful of members to actually want to compete against each other.
I ran a similar contest recently, and withdrew it due to a lack of interest.. Then again, CodeForum.ORG has significantly more members.
My initial thought would be that 25 and 50 referrals respectively seem like an unlikely target for a "contest", and the reward doesn't tally with the effort involved.
Leaving it open as the member with the most referrals within X amount of time seems more feasible.