Well there are a few reasons:
Because I would make him cradle to grave. Getting the customers in the door is now the hardest part about selling a car. If he can maintain his percentages with 130-150 leads, that's 19.5 cars. He would make 70-$100,000 /yr depending on gross, keep his normal schedule + have off every Sunday [Charles county blue law, dealers are closed]

Now why in the world do that??? You just said that getting people in the door is your biggest challenge. Why would you dilute a propensity for doing just that???
You make him cradle-to-grave, at MOST he can handle 100 leads a month, that's 25 shows and hopefully 15 deliveries. How does that move the needle?
Play to a strength if you want to move the needle! This man gets people in the door!! Pay him to do just that -- work on increasing his show rate.
If it's me, I cut his leads to 150 and pay tiered bonuses at 40% show and a whopper at 50%. At 300 leads he's forced to cherry-pick now; imagine what the right attention and process for all leads would produce.
Thats how you move the needle!

By the way; no Sundays ever at Duke, and every other Sat off. And rocking those numbers, he'd be my new BDC manager with bonus levels that John just described for shows for himself and the entire BDC performance!