Exactly. After a couple years of reading the slantsix.org board and listening to SlantSixDan's advice, I have upped my mileage from 10 to 23mpg. Not only that but my slant has more power as well.
If he wants to be stubborn then let him be stubborn because everyone and their mother knows getting over 16mpg highway with a sl6 is easy unless you're in a big truck or van.