Anyway, getting to your main question...As I'm sure you're aware, there are a lot of folks who have very strong opinions about the best foods to feed dogs. Just recently, we've had several folks join us who have told us that they've read that raw diets are best for Cushpups, but I do not know where that info is coming from. I am unaware of any documented scientific studies that have established that any single type of feeding choice is better or worse for all dogs with Cushing's. Such studies may be out there, but I've not yet come across them. Of course, people do share their opinions and anecdotal experiences on the internet, though, and for what it's worth, here's mine.
In honesty, I can't say that during my decade spent on Cushing's forums, I have felt as though any particular style seems more suitable or more helpful for Cushpups. I am not a fan of raw feeding myself, but we do have some folks who feed raw food. Other folks feed the whole range of commercial wet, commercial dry, and homecooked. We have seen dogs who have done very well on each type of food, and dogs who have had problems on each type of food. So my personal, nonscientific opinion is that there is not any single choice that is generally better or worse -- it all comes down to each individual dog's GI system and constellation of health issues.
The general recommendation I do often make is to serve a food that has a moderate level of good quality protein, and also lower levels of fat. Cushings can affect the kidneys in such a way that protein gets spilled into the urine. If a dog experiences significant urinary protein loss, then it's probably best not to include high levels of protein in the diet. The rationale for the lower fat is due to the fact that Cushpups may have a higher vulnerability for developing pancreatitis, and higher fat diets may heighten this risk for some dogs.