Using that logic, why bother putting aftermarket differential covers on that allow increase lube capacity AND increase the cooling of the lube by the heatsink effect of the cast aluminum with internal and external fins? Increasing the capacity of the transmission 3-4 quarts with a largernpan has more benefits than just temperature reduction (although the Derale pans DO demonstrably lower trans fluid temps 20-30ndegrees with their pass through cooling tubes) such as reducing the concentration of suspended contaminants in the fluid. Following your logic further, both the 6.5 and for that matter your D-Max don't need anything larger than a 4qt oil pan on them. Are you willing to reduce that capacity? Why do racing gas motors run 7 or 8 quart capacity pans, instead of stock 4qt, in addition to massive coolers?
No hate, just asking for the reasoning/logic in not increasing both capacity and passive radiated cooling of the transmission by going with an aftermarket cast aluminum or steel pan.
It's one thing when you add in a pan that is like a heat sink, but to many people believe simply adding capacity increases cooling. Cooling is done by heat transfer, so unless you increase the area for heat transfer, you've done nothing to help with heat. And for a V-8 engine where the valleys, covers, oil passages, and such of the engine can easily hold 3-4 quarts of oil, only having 4 guarts don't make much sense. You can bet your but GM didn't make the oil capacity any larger than they had to. Although many HAVE decreased the oil capacity of the DMAX engine down to about 5 quarts, with NO ill side effects. If you want to spend the money to add in a pan with fins and heat sink area to help with heating, that is fine. I personally feel that money would be better spent on added cooling where you can buy a larger cooler for less money than one of those fancy cast aluminum pans to aid in cooling. As for contaminent loading, added capacity can help with that, but only to a certain degree. Filtraition is key as I would rather have the contaminents held in a filter media than I would to try and hold it in suspension in the fluid. And if your getting that much contamination in your fluid between services, then you have MUCH bigger issues than worrying about a bigger pan.
You can look at it however you like, but larger pans have proven little to no benefit. If they did, the OEMS would have put them on to begin with. GM did increase the pan capacity some in later years, but other changes also took place to increase the amount of fluid in circulation, and I believe those changes neccesatated the increased capacity more than anything lese. MANY people have added in bigger pans, and complained that there temps still got just as hot, it just made it take longer to get there. IMHO increasing the cooling and filtering(if your concerned with fluid loading of debris) is money MUCH better spent than the kind of money most places want for a pan that has minimal results in helping with heat shedding. As for your example of diff covers, yes that can help since they have virtually nothing to begin with. So if you add something to nothing, of course your going to see semi drastic results.
If I thought for one minute simply adding fluid capacity would help, I would have a larger pan on my BURB. But nobody has proven a pan to be as effective as an improved cooler, so I went with a premium cooler since that method is proven to work for less money.