How much will track spikes help your mile?

It depends. Simply wearing track spikes is not enough to make you a good runner. Conversely, it's possible to run a very fast time in ordinary sneakers. Racing spikes are just shoes designed specifically to wear when racing, they are a lot lighter than ordinary trainers but have less support. Personally, I've found wearing track spikes can have as much of a psychological boost as a physical boost (since you're wearing lighter racing shoes you feel faster). If you're serious about racing you should probably get a pair of spikes but if you're just running the PE mile you can do all right in normal sneakers.