Just heard back from a trans rebuilder that said cooler line pressure is indeed different than line pressure. He said cooler pressure is probably 65psi max. He uses compression fittings in that circuit all the time.
The Earl's sales guy responded back and said the tube nut/sleeve would be the safer option. I guess it just depends on if you are comfortable with the compression fitting. I've got some thinking to do. A friend has a 37 degree flaring tool, so re-flaring the tube wouldn't be a big deal.(as long as an aluminum tube nut/sleeve can be used on a SS tube)