The local Chevy dealer says that they recommend a tranny flush at 20,000 miles and they only use Dextron III. They charge $299

Question: Does the dealer come close to cleaning out the ATF as Hoots method and do they flush by accessing the fluid at the cooler return line?

If you do it yourself using the "Hoots" method you could use Transynd and still save money. Allison says not to machincine flush these as it may cause the clutch plates to delaminate. This comes directly from a sevice tech at Allison. Also you would be safe just to drain the pan, refill with new fluid, and change the external spin on filter. A flush at that time is not needed, he probably just needs to buy gas for his car and what better way than to have someone who trusts him pay for it.