The canned tunes can only operate in a very save range for the changes made since they are generic for any truck. A custom tune, which you know, reads the actual parameters of a vehicle and the tuner can tweak things much farter. But as Gary mentioned, a hug benefit is the trans tune he does. When I started with the canned tunes, shifting was similar to stock where it hunted for gears in situations like rolling then getting back on the gas. And the canned tune shifts at heavy throttle were way way to harsh. Felt like I was going to drop the trans out. Lew's tune is very refined...so much better than stock and yet hits fast and precise when you get on it.
Its like soft butter on warm french bread...tastes and smells devine...and...smoooooooth!