Nick has the ticket. The Chaos C5s. Just on the basics of fin designs, they should be faster than the original C5, which wasn't a great fin to begin with. I've yet to try them since I hung my Walzer up and I have a new ride but, I am going to get some just based on principle. It's the first decent fin for that C5 box as far as I know.
I've got 4 or 5 extra sets of the originals if you can't find them. I've also got a set of modified FCS that will work in a that box too but, I didn't like them. If you're passing through Reno on your way, I'll just kick you a set.
http://shredstixxusa.com/wakesurffins/chaos-c5-fin-pair