Hi, I was just wondering if you need to be a certain speed to run in spikes or flats in order to get the most out of them. Because I have about a 19 minute 5k and 5 minute mile, and I was thinking of buying the Nike Dragonfly's for track since I mainly use the Nike Pegasus for daily runs I'd want a good one for track and possibly XC as well. But I was wondering, am I fast enough to use the Dragonfly's? Plus for XC, is it better to get the Dragonfly XC's or the Streakfly's, as I have just been using my Pegasus' for XC and I've gotten a 19 minute 5k. Will spikes or flats really be beneficial to my running to decrease my times by a lot?