Welcome!
The most likely problem isn't insulin resistance, it's frequency of testing. I am guessing from what you don't say, that your blood sugar tests are taking place at the vet's, and at some undetermined time after the last shot.
A typical cat after an injection of insulin has their blood sugar go down for a few hours, then come back up again over a few more hours. 12 hours later sometimes it's as if the insulin wasn't there at all.
Different insulins have different typical lengths of action, and each cat has a different reaction too. So although PZI typically lasts about 8-10 hours in cats, some make it last a full 12.
Humulin N typically lasts more like 5-7 hours in cats, and makes a rather sharp dip in blood sugar, often dipping to a dangerously low peak around 3-4 hours, before rising rapidly again. If the dip is too deep, it can activate the cat's liver's own defenses, dumping glucose into the bloodstream and rapidly raising blood sugar even higher than it was to start with, which might explain your test over 400.
One more important thing is that stress (such as that of being at the vet's) causes cortisol and epinephrine to be released into the blood, which both quickly act to raise blood sugar. In some cats this effect is minimal, in others it is dramatic.
So your vet's conclusions about insulin resistance tell me he or she is not experienced with cats with diabetes. There are many things you can do to help out.
1. Get an inexpensive human glucometer from the drugstore and start testing your cat's blood sugar at home, frequently. (You don't need to do it more than 2 times a day in the long run, but for now you need more data to decide what to do.) It's easy after a few days, see this:
http://www.youtube.com/watch?v=_zE12-4fVn8
2. Switch back to PZI for now (Humulin N is too fast acting and short-term for most cats), and start with a lowish dose (0.5 units) and gradually increase by no more than a half-unit, every other day.
3. Use a spreadsheet such as (can someone please link to the google docs template?) to record your blood sugar tests. We can help you figure out how long your insulin is lasting and how effective it is, and suggest when to change dose.
4. Others will chime in on diet recommendations. These will save you lots of money and make regulating blood sugar possible, and are very important.
There you go, breathe easier, you have found a lot of experienced folks!
Best,
Steve and Jock