I have never seen a larger intermediate or tail pipe lose power or torque. I have seen it happen at a certain point in the curve with too much primary pipe (header tube) or too short of a collector with open headers, but not after the collector with a full exhaust. In fact, many of the techniques, X pipe, H pipe, and termination boxes, try to fool the exhaust into thinking it has escaped/dissipate the pulses, creating essentially a "huge or infinite ID". (I am no engineer, so if I murdered that last part, correct away)
In fact, even the lowest compression, laziest V-8s seems to make at worst no gains, or improve, with "too big" full exhaust if the headers are sized right. In my experience, cost and body fitment drive the exhaust size. My technique is to use headers that match the build, then run a mandrel bent exhaust the same size as collector. Additionally, muffler design is important too, and modern absorption muffler like Hooker Maxflo, Magnaflow and the like, tease a bit more out of it as well.
That test doesn't surprise me, but does surprise how much of a difference. FWIW, my 8.5:1 270H 396 woke up going from 2.25 to 3 inch, but it's hard to say if it liked the ID or the bends as I went from 80s crush bent to modern mandrel bending too. Admittedly, I didn't build that much exhaust for the tired 396, I did it in prep for the 445, but it worked out better everywhere even with the old engine, and that's a 4400lb truck with 33 inch tall tires and 3.50s, not a high RPM screamer
