Can I ask why the low fat? Fat usually, with the exception maybe exacerbating some of the of weird liver values Cushing's creates, doesn't become an issue b/c of Cushings. Food issues aren't a general issue with Cushing's, if you factor out the incessant hunger when the disease isn't well controlled. Some drugs interact with certain foods (ex. Anipryl) so that's a consideration but it's always a consideration with any drug. What are you doing for medical management?
Max, our Cocker has pituitary Cushing's (Diagnosed 10/2015) and we manage him on Primal raw but it's not inherently low fat (crude fat min. 14%) and he get's plenty of 80/20 ground beef plus fruits and veggies and egg for calories. He was on NV Raw for awhile but several years ago they changed his formula and it no longer worked (long story very very short). I'm sure you know Cushing's has a tendency to cause weird muscle wasting in the spinal ridge and weird pooching in the groin, among other symptoms when uncontrolled, so I try to make sure we have weight in appropriate places. My experience is to feed high quality food that provides the calories the body actually needs in a form it can use, (not the amount of calories the excess cortisol thinks it needs, he'd be 50# instead of 24!). Recently we did notice Max was too thin and upped his food portions, not that he minded one bit.
We have other medical factors to consider as of late so our recent experience is somewhat predicated on that but we haven't changed his diet formula since the Cushing's, with the exception of deleting any foods his meds interact with.
The last time I checked Wellness was still a reputable brand but I haven't fed it to any of my fur faces for about 4 years when we had a cat who ate the canned b/c of kidney disease linked to old age. If THK is generally agreeable but it just needs more fat, could you add some fatty ground beef or some fish oil?