Of course... Size (and quality) matters when it comes to cabling. It's simple. The better the wire, the better the signal and overall inherant performance from the completed installation. Not to mention fewer problems related to wiring in six months, a year, or 20 years.

If you traffic a load meant for a 4-lane highway down a 2-lane road over time, that road is going to degrade. I've seen it with my own eyes, (Cat5 running stuff it shouldn't)and seen and heard the results after fixing wiring jobs that were installed by people who think the same as your friend. Those customers wasted money on having the work corrected. The problem is not the customer however, it's the contractor that sold them the wire in the first place that should be blamed. I'm shocked that an E.E. would have this opinion about such a basic thing as physics.

Though he does concede that the signal would only be different, and not better, is certainly an indication to me that he knows better and he's just trying to get a rise out of you.

In my humble opinion, there's no substitute for doing it right the first time.