I have had detailed personal conversations with Dan Lilja about this very subject. His feelings were basically the exact same as what your friend told you he saw.
That said, while in a 223 or 308, you will likely not see much difference in pressure or velocity change but when you step up to a chambering that is really extreme, this insignificant change in pressure can become quite significant.
For example, and this is only an example, NOT TRYING TO SELL RIFLES
, if I took my 7mm AM with a 160 gr Accubond loaded to 3550 fps, which is a top end practical load, in a 1-10 twist Lilja barrel and then shoot that same exact load in a 1-7 twist Lilja barrel, the faster twist barrel will show obviously higher chamber pressures.
Now if I take that same 7mm AM, load it to 3400 fps and repeat the test, you will hardly notice a difference between the two.
Point being, if you run an extreme round to the red line, you will see MUCH more diffence when small changes are made in things such as bullet weight, twist rate, groove number, groove width and even things such as jacket thickness. In most chamberings these things are pretty much "insignificant" as far as increasing pressure but in extreme performance chamberings, that is not the case.
Another good reason not to load to red line!!!
For conventional chamberings loaded to conventional pressures with conventional weight bullets, I would agree 100%. With the extreme end of chamberings, it can be more noticable when you make changes like this.