3. Jarate is not always useful, especially in situations where teams are small. The SMG is the Sniper's best weapon for self-defense, and great Snipers can even make their own picks with it against careless ambushers.
5. Don't always aim for the head. If you happen to have a full charge and you see a Scout or any other 125-health class rushing towards you, it's absolutely stupid to aim for the head.
7. Pro Snipers don't track, they twitch aim. Tracking only makes sense on weapons that deal out steady streams of damage. If you track with a weapon that requires precise, singular shots, tracking is ruined by the slightest variation in the opponent's movement. Twitch aim allows you to immediately target the point where you expect the opponent to be without having to compensate for longer than a split second.
Also, stock is not the best rifle by a long shot. In small team situations, the Sydney Sleeper provides much more bang for the buck, and, if going against another Sniper, the Croc-o-Style kit pretty much puts you on top for very little cost.