Can you give some additional details regarding diagnosis? Were there symptoms of diabetes, like excessive peeing and drinking a lot of water? Other than the BG test, did the vet run any other tests like a fructosamine, or urinalysis? 16.45 mmol/L is 296 mg/dL, so that's higher than normal, but some cats can have stress hyperglycaemia (elevated BG level as a result of the stress of visiting the vet), so having a few other tests to verify like fructosamine and urinalysis would make sense. If there was a urinalysis, it should indicate whether glucose was present in the urine, and as well, please check whether there were any ketones present.
In any case, those tests you took, when were they in terms of hours after insulin?
Also, what are you feeding? Any dry food in the mix? We have found that dosing twice a day is best, since cats metabolize insulin differently than humans, so I would say going forward, it would be better to give 1u twice a day, instead of 2u once a day; however, it would be good if you could clarify some of the questions above. 1u twice a day is generally a good starting dose, although if you recently removed dry food, we would need to know that as well.