I've seen cats be high like Toby before, peg. Punkin was like that too - the whole summer of 2011 I increased his dose and nothing happened. For whatever reason, it seems like inching up the dosing scale will often result in the same numbers until you suddenly get to the dose the cat needs. Then the action all starts. So don't be discouraged that he's staying high as you go up the scale - there will be a dose that moves him!
Here's a spreadsheet of a kitty that is a good example of
that. What I would point out is that this kitty's blood sugar stayed basically the same as the person increased the dose, per the protocol, until kitty got to 10u. Then there was a sea change, and a lot of green appeared.
This is a bit of an unusual situation, because this person's vet was very unhappy that the person was increasing the dose per the protocol, against the vet's wishes. Right about the time the cat started responding the the 10u dose, the vet basically drew the line and said in order to remain a client, she had to follow the vet's dosing directions. So she dropped out of FDMB and has obviously tried a couple of different things, including switching insulins. It's likely this cat has something else going on - if i recall correctly there was a thyroid issue. It's possible that there is a high dose condition too.
The point that I thought was worth pointing out to you was the first half of the spreadsheet when she just kept increasing the dose and the numbers didn't budget - until she got to 10u. It's very counter-intuitive to think that increasing the dose won't change the blood sugar along the way, but that's often the case.
Now I'm not saying Toby needs 10u! Just making the point that sometimes BGs just don't budge until you hit the right dose for that cat.