Well, you have a couple of unknowns that could be affecting a "breakthrough." If I recall you haven't had Toby tested for acro or IAA, right? Given that he is on 18u of insulin, I'm guessing most people assume he has a high dose condition. And trust me, I know how irritating it is to hear "you need to get him tested." I heard it from day one and it took us, what, 6 months to have him tested. We were at 21u of Lantus when I finally had him tested. At the time I didn't have a job, had a new baby and we really just didn't have the money to do it. Most everyone here was accepting of that reasoning and we proceeded assuming Cobb was an acrocat. We ultimately didn't do the acro test (for various reasons), but confirmed he had IAA. But again, we proceeded with our treatment as if we knew Cobb had acromegaly...nothing in the treatment plan changed really. Give enough insulin to get BGs in a normal range, reduce if earned, increase if needed.
If, and I say "if" because I hated it when people wrote in a definitive "Cobb has acro" terminology (perhaps a bit of denial on my part), Toby is acromegalic, then the amount of insulin you are currently using is what is needed to offset the drip of hormones from the pituitary gland. And until that drip slows, you won't see a need to reduce because 18u is what Toby requires for that offset.
If you had Toby tested, you would have an answer to that "why aren't we seeing a breaththrough" question. Is it worth the expense so you aren't left wondering? Only you can decide that.
I'm pretty sure that isn't really the answer you wanted, but I think it is rare (although not impossible) for a cat who just has diabetes to require such a large dose. Even cats whose pancreases are very damaged don't usually require that much insulin. I would bet there is more going on than just "plain ole feline diabetes."
