Yes, 300 is over the renal threshold. There is some disagreement in the various vet journal literature on where that renal threshold is, generally considered somewhere in the 200-280 range for a cat.
While your vet may be happy to see your cat in the 300 range, being over the renal threshold will cause slow but steady nerve and organ damage to your kitty. So for that reason, we recommend having the kitty below renal threshold as much as possible. Of course, it's not always possible, but you do the best you can.
While your vet may be ok with having your cat "regulated", the latest goal for a diabetic cat is diet controlled remission. That goal is in many vet journal published articles for at least the last 6-10 years.
We sure do hope we can help you to get your kitty Noodle into better BG levels and maybe even that diet controlled status.
Keep asking questions and we'll help as best we can. Lots of experience here in the day to day management of diabetes.
Hope that new vet works out better for you and Noodle and is willing to treat your relationship as a partnership in Noodles care.