Not sure I agree with this for a 40 anymore.
All of the newly manufactured 40 STI tubes I have received (at least 8 in the past 2 years) have all ran out of the bag by simply dropping in Grams followers/springs with a base pad of my choice. They all fit hold 20 and fit the gauge as well. They run no different than my 4 "tuned" Brazos mags, which also only hold 20 comfortably/reloadable. Feedlips are all within spec as well.
Have someone tune them for 21+ maybe, but for 20? I don't believe the hype anymore.
I find it hard to believe I got lucky with 8 different tubes bought on different occasions.
You have had better luck then I have then. I had an issue with one of mine grams guts and TTI base pads. The others were good.