Seriously? I have a stock 5.9 with bolt ons and I am leaving a huge amount on the table by running a stock tune. That is a ridiculous statement. A bone stock Magnum was horrible in stock form. Plain and simple.
Well for me it works. Like I said its not ideal, but it works. I use it as my daily driver and I'm getting around 14-16 mpg. Not much different than the 5.2 that was originally in there. But it has a lot more power than the 5.2.
If he is doing a stock rebuild with the 5.2 then he will be fine.