...and what a difference. I've spent lots of time trying to figure everything out. I learned that my Michelin Power Cup 25 mm tires are actually 28.5 mm wide when fully inflated . I actually weighed my bikes. I was riding a late-90's Scapin Pro Racer with alloy wheels that have a 17 mm inner width on very smooth roads at about 90 PSI. Felt fast. But my little bike computer told me a very different story today after I inflated to 76 rear and 71 front. The acceleration didn't feel as fast, but I was really rolling once I got going. I have to get more scientific, but it appears that I was hitting speeds over 2 mph on certain parts of my ride more than when I was riding higher PSIs. Oh, and the practice of placing the hands on the hoods, lowering the head forward, and compressing the arms/elbows inward made a huge difference. You can see the speed jump when you get into that aero position. Really amazing.

I honestly think that would feel mushy to me in the corners.

Kudos. As I have been saying for YEARS, most novice (and many “experienced”) roadies are overinflating their tires.

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I run tubeless 25 mm tires on both my regular road bikes at ~ 72 front and ~ 77 psi rear and this is just about right and not mushy in the corners.

Madness!

Big Low pressure makes another convert...

If your data set contains two changes, then you are going to have to do some further testing to figure out which was the more significant. I might lean more to the aerodynamics being the main contributor over the inflation pressure.

But 12 of the same rides for each different variable might convince me the other way. Hmm, 36 rides.... get going!

At about 220 lbs. system weight, I previously ran 25 mm Continental GP 5000 mounted on wheels with a 19 mm internal width at 85 PSI [F] and 90 PSI [R]. This was probably slightly high.
A month ago, I upgraded to new wheels with a 21 mm internal width and kept the same tires, but initially at the same inflation pressures, purely out of habit.
A couple of rides ago, I lowered the inflation pressures to 80 PSI [F] and 85 PSI [R]. The rear end felt slightly too soft on acceleration but handling noticeably improved, especially on fast descents.
On my last ride, I further lowered the inflation pressures to 75 PSI [F] and 80 PSI [R]. Felt about the same.

That's awesome! It’s crazy how much small adjustments like tire pressure and position can change your ride. The aero position really makes a difference once you get used to it. Sounds like you're dialing it in!