Neko spend some time in hot weather. When it was so hot we hid in the house, she head out to our hotter sunny solarium. And she had long hair! Crazy cat. No impact on her BG's.
Have you considered going up by 0.5 units instead of 0.25? Most of the greens are upper greens. But it depends how much you can monitor the next couple days. He also might break any bounce today or tonight so might wait until after that. Go for the smaller increase if he sees green again today.
One thing we mention to caregivers when their cats get to 6 units of insulin, is testing for a secondary endocrine condition. In the past they were called high dose conditions, but we've had kitties with these conditions on smaller units too. The conditions I'm specifically thinking or are acromegaly, IAA (insulin auto antibodies) and Cushings. Those are listed in order of most common. Neko had both acromegaly and IAA. Acromegaly is caused by a benign tumour on the pituitary that sends out excess growth hormone, which in turn causes the diabetes. IAA is like an allergic reaction to the injected insulin. Cushings is caused by either a pituitary tumour, adrenal tumour, or excess steroids. The tests for acro and IAA are blood tests send to Michigan State University. I'd do those first. 1 in 4 diabetic cats has acromegaly according to the latest research. There are treatments for acromegaly. IAA goes away over time.