Well, my first question to that vet would have been how many diabetic cats has he treated? I'd also be very curious what symptoms he's observed that would lead to that diagnosis. My guess is that the vet is thinking of an insulinoma. Boops' numbers are not indicative of this form of pancreatic cancer -- you would see low numbers and episodes of symptomatic hypoglycemia (collapse, loss of consciousness, seizures, etc.) There is a saying in medicine, "If you hear the sound of hooves, think horses not zebras." Horses are a lot more common than zebras. In other words, don't jump to a conclusion that is a rare occurrence. Go with what's the most prevalent condition and until that's ruled out, don't go to extremes. In the absence of knowing what led the vet to this conclusion, it seems like this vet is going to extremes or is inexperienced in dealing with feline diabetes.
You might try picking a doctor you like and at least for routine visits, only ask to be scheduled with that vet.