Yes, the grill shutters suck for trying to being down thermal deltas. I have been fighting this issue myself.
If you get a tuner (HP or Diablo) you can simply disconnect and remove the shutters. Once you do this you will receive a fault code in the engine along with a check engine light. With HP or Diablo, ask the tuner to simply make that P code "not reportable" and boom, done. Once I am better, the grill shutter delete is my next objective.
I have some data somewhere but I think it was 35mph where they shut (obviously dependent on parameters) and then they opened again as I was coming to a stop. Even with my 174F katech coolant thermostat, I was still seeing 190s + for ECT. Once the shutters opened my temps dropped.
Ditto on the trans temps. Much better with a pill flip but summer heat in AZ and towing can sometimes get up there a little