Reduced IAT due to increased heat exchanging ability should show gains even at stock boost levels, especially during long WOT high speed pulls. It would be interesting to see the gains if any with just the intercooler, and then with each next mod one by one.
And agreed, higher IATs due to greater pressure and more turbo rpm than stock should showcase the extra heat exchanging ability even better.
I woldn't consider a dynamometer readout to ever be a sales gimmick, unless the results were skewed or modified.