Generally any dose over 6u is considered high although I've seen some vets suggest not testing for high dose conditions until they reach 8u. I've never seen a reference to any particular insulin so I believe it applies to all insulins. I've also seen 1u per pound of body weight as a reference so if kitty is large, they may need more insulin without being suspect for an underlying condition.
When kitty is on 1u of insulin we increase or decrease by 0.25u which is 25% of the dose. When they get to 5u of insulin an increase of 0.25u is only a 5% increase which is fine if you are finessing a good dose but not helpful in an unregulated kitty whose sitting in high numbers and not seeing much movement. The problem is that the longer they sit in high numbers, the more difficult it is to get them down so you need to be a bit more aggressive when dealing with the higher dose kitties. There are some cats on copious amounts of insulin so full unit or even larger increases are needed to get ahead of their numbers. There have been cats here on 50+units of insulin.
Pati, Morris is getting some good numbers mid cycle and you seem to be honing in on a good dose so if I were you, I'd probably stick with 0.25u increases even if you do hit 5u or just above so you don't miss his "ideal" dose.