Is there any reason this could be harmful?

At this point if your horse is doing fine on the supplements that you are giving-then let it be. Higher doses of anti-inflammatories can cause problems in their own right. Discuss this with your vet. As some one else posted you may need a stronger supplement in the future and that is when you would probably turn to this. When adding or changing my horses supplements I discuss it with a vet or the horse nutrionist.