All ITSPs then should ditch SIP for PSTN trunking and move to support IAX2. A much simpler protocol and it goes through NAT like a knife through butter.
The issue with IAX2 is that currently you are limited to Asterisk and its forks. I currently have Sonus and sansay products in my network and IAX2 is not an option. I agree that IAX2 is great for nat but have not seen any studies on high volume/high CPS. I peak during the day at 6k cps and 150k channels. You would have to get the big vendors to buy in and they never agree on anything.