From my limited cat 5 experience in 2010, I'd second this. My first proper Cat 5 event was a hill climb, and the winning 5 time was good enough for 3rd in the 3's and mid pack in the Pro/1/2 field. I then tried a handful of crits - no idea why I ended up doing that, and it was a blast. Definitely get some pack riding experience or do a mentored event first though - it'll build your confidence and really improve your riding. Managing the surges seems key to me.