This was on a 104" engine with 11:1 compression, .495s, bowl blending/polished valves/multi angle valve job, RPW Slash (SlashNess) pipes, and PC-III.
Blue lines S&S airbox... Red lines Torque Tubes... no other changes, just a re-tune.
Gains at peak were 2.3 HP/11.76 ft-lbs. The biggest gains came at 3400 RPM where it picked up 11 HP/17 ft-lbs.
They're not called "torque tubes" for nothin'... 107 ft-lbs @ 2200 RPM.
The reason for the shape of the graphs (flat until 3200) is the pipes.