If you are considering stopping the insulin, whether you stop it cold turkey or back the dose down slowly till he's off it, the result would be the same. Assuming the diabetes diagnosis is correct, Do Daa could develop ketones and DKA or he might simply have a very prolonged decline and eventually death. High numbers for a prolonged period of time will cause damage but cats are remarkably resilient animals and it takes a long time for damage to occur. There are secondary conditions which sometimes cause a cat to need more insulin than most. My girl, who is now in remission, had IAA and was at one time on 16u of insulin twice daily and she by no means holds the record for big doses. There are tests that can be done to rule out those high dose conditions but they usually aren't done until the cat gets to a dose of 5 or 6u twice daily. While most cats don't need large doses of insulin, a cat needs the amount of insulin it needs. It's possible ProZinc isn't the right insulin for Do Daa and a switch to Lantus or Levemir might work better. It's also possible he has developed glucose toxicity and is going to need more insulin for a time to get past that and then the dose will decrease. This is a marathon not a sprint and you are still in early days of treatment. I urge you not to give up.