I have Rummy on buteless, it doesn't have devils claw in it as he has cushings, but much the same thing I would imagine. I noticed a real improvement in his movement from a week on it. He is now on it just over a month and he is lying down in his stable at night as he is covered in shavings in the mornings , I was really worried about him as I hadn't seen him lie down in a wee while. So it definitely is helping him. I had Rummy on 10mls twice a day at the start and now he is on 5mls twice a day and he is only about 11hh and weighs under 200kg
I have edited this to add that I just looked at the ingredients on buteless and its not the same as yours so you can just ignore all the above. Sorry