I think he means you're leaving a lot on the table with a 6.9 pump on a 7.3, not lean in that it will kill it like lean seizing a gasser. There's validity in his point. The 6.9 pump uses .0029" fuel plungers, the 7.3 pump, .0031" and the John Deere DB4 uses .0033". That said if I had a stock 7.3 truck I'd throw on the 6.9 pump and not expect to see much of a performance drop.