Yes, the mechanic AND the Father in this instance are wrong. Why? Simply because we have more experience with this system than they do. Nobody is stupid here, they just don't know why it'* an improvement. You have a collective experience level on this forum of several hundred years with the H body GM cars.
You will NOT see a decrease in fuel efficiency, but rather a possible slight increase.
You will NOT see a full 15° difference in operating temps. It'll only drop the average by 5-10 at the most.
You WILL see a slight improvement in performance and throttle response (hardly noticeable, but remember how your car always seems to have more power on a cold day?)
You will NOT cause emissions problems. If the Grand Prix'* in California can get away with this and still pass the strictest emmissions standards in the country, there'* no issue.
You ARE more likely to crack the upper intake on an L27 or L36 if you don't do this.
Don't bust your *** trying to convince someone to do this if they resist it. Wait for them to crack the upper intake, then tell them "I told you so!". The more you argue with someone, the more resistant to change they become. It'* a human trait.