I don't consider Slughorn to be that perceptive a judge of talent. Snape was a prodigy at spells from his first year, long before any association with Lily
Also there's the matter of the Levicorpus levitating spell that Snape invented. In HBP, Snape's book says it's a non-verbal spell. So how would James have found out enough about Snape's non-verbal spell to be able to use it against Snape?
This is my scenario: Snape was infatuated with Lily, and helped her with her coursework. In the course of this help, Lily learned all the spells that Snape developed. Lily falls for James. Lily tells James about all Snape's spells, and James uses them against Snape
If someone you loved left you for your worst enemy, and betrayed all your confidences to your worst enemy, how would you feel about that person? Especially if you then decided that person was only using you all the while?