Doesnt matter if you are bone stock or have 600hp and a 4000 stall converter...the 545RFE and 65RFE do not handle WOT downshifts very well at all. I know more than 200 people from stock to modified that have blown their transmissions because they let it automatically downshift out of overdrive on the highway.
Alright. First off, where is the proof to back up this claim? This is factually incompetent gibberish. If it were true that actual owners were using these transmissions as designed and they failed as consistently as you say they do, Chrysler would no longer be in business. Period. That or at the very least, they wouldn't be putting these transmissions in anything. We're not all idiots and non-mechanics in here.
I've owned 60+ cars, from classic street rods and muscle cars to new pickups to family cars to factory performance cars and everything in between, foreign and domestic. I normally only shift manually for engine braking down mountain passes, especially if towing and on the rare occasion that I'm messing around burning rubber on the street. I have never... not even one time had a transmission problem in any of my vehicles. I properly maintain them and don't abuse them.
For someone to claim they "know" over 200 people is a stretch to begin with. For someone to claim they "know more than 200 people from stock to modified that have blown their transmissions because they let it automatically downshift out of overdrive on the highway" is just plain ridiculous babble! Especially when you narrow that down to two specific transmissions.
If you can show me one quote from ANY auto manufacturer OR transmission manufacturer that says you should not allow the transmission to downshift automatically from highway speeds, or any speed for that matter, or damage will occur, I will concede. That, or any actual service write-up, warranty claim, TSB, or anything official citing that this has actually happened. You did say it didn't matter if it were stock or not. Otherwise, you have lost all credibility in my eyes. As a vendor, you should choose your words carefully. People like to think they can rely on you for credible information and sound advice.