A/F Ratio's compared
 

Had my car dynoed (car is stock w/ air lid). My A/F ratio was pretty constant at about 13.3:1. I was looking @ another guy's dyno sheet and he out horsepowered me by 10-20 hp @ each rpm above 4K and his A/F was more like 12:1. Does my car seem to need a tune?

He would probably pick up a bit leaning on it some. You, on the other hand most likely will not do anything but loose HP by richening it up. You may not loose any, or much, richening it a tenth or two (.1-.2) but anything more you will almost assuredly start dropping.
ALL engines will make more power the leaner you can run them, until the point of fuel deprivation. The trade off is running them too lean, and causing durability of longevity issues.