Princessk, you don't need to feed an special, fancy, expensive vet food... I am not sure said you did, but anyway, you can feed ordinary store bought pate food. Most people in the US and Canada just feed Fancy Feast pate or Friskies or 9Lives, .... you will know if the food is causing your cat's numbers to rise easily..... so long as you are home testing, pick up an ordinary blood glucose meter like the OneTouch or Aviva, or Accu-Chek, the test strips for the meter, some lancets and a lancet device, you can test at shot times, and then an hour after feeding to see how much the food caused your cat to rise.
I don't know if I missed it or not..... what insulin are you giving? Since you are at a higher dose, you may want to be using Levemir (detemir) as Lantus has been said to sting, that's according to what some humans have said, and if you are using some other type of insulin, what is it?
Sure there's a test for acromegaly!
Here's the link for the test we have done in N. America to get the IGF-1 value
http://www.animalhealth.msu.edu/Bin/Catalog.exe?Action=Test&Id=1401
I had 2 cats positive for feline acromegaly, and the vet I went to at first knew nothing about the issue or where to get the test done. I had to print out the above sheet and take it to the vet! It's taught in vet school that it's a rare condition, so students get nothing in the way of education and so, why would they know where to get the test done? Oh well, we can get angry at them or get them the info and say do it!
Now make sure you are home testing as that's the only real way to get true numbers from your cat to see how the insulin is working, and if the food change makes a diff.
Maybe if you show the vet the above link for IGF-1 testing, your vet can ask where blood is usually sent for testing.
Your range is diff though.... maybe over 1000 is positive, or maybe it's 2000. Anyway, keep bugging your vet.
Most vets are pretty lousy at nutrition for animals, and no surprise your vet is like others.... all about the money by pushing lousy pet foods.