On that engine, gains are typically in the 25-30hp range and even at the 5000-5500lb range that it is, they can still be very noticeable. You won't only see improvements at full throttle, you should see efficiency/power improvements at all load points.
The 01 has the same transmission as all 01-06 Denalis, which is a 4 speed based on the 4L60E platform. Many of the response improvements can come from transmission tuning on that particular vehicle, as the factory shift points, both up and down, are very lack-luster in timing and pressure.
Regarding the difference between our tunes and any handheld, those differences can be significant, especially with our scan cable, AutoCal or in person option. Most handhelds on that model make less than a dozen changes based on one or two test vehicles that may or may not have been running in ideal condition at the time of the tune. Our tunes are built based on thousands of past vehicles of similar build and are custom tailored to your specific vehicles needs and often, your usage.
No handheld can even properly compensate for something as common as a cold air intake, let alone more extensive modifications.
