When a
mechanic sacrifices his Saturday and comes to a
tuning day to babysit the cars he works on when they go onto the dyno (including one with a turbo that was rebuilt about 16 hours before getting tuned) and does quality, well reviewed work to boot, you don't even need to be an current customer to know he's going to earn your business going forward.
Not to mention the mechanic's current customers, who are fanatically loyal.