When Gabby was first diagnosed and before I arrived at FDMB, my vet suggested I feed Purina DM (canned or dry) or Hills MD. I first found a website authored by a vet (Lisa Pierson, DVM), that discussed
feline nutrition in depth. I went back to my vet's office with cases and bags of food and asked for a refund. I explained to my vet that I could find food that was equivalent or lower in carbs and at least as high in protein that was better quality and cost less. The vet didn't argue. She knew that I understood what the purpose of the food she prescribed was. (If you look at the ingredients of DM, it's primarily animal by-products.)
Janet & Binky's food charts will provide you with a lot of choices for foods that are lower in carbs and better quality than DM.
The feline nutrition site that I linked above also has very good information on how to transition a cat from dry to canned food. When you decide to transition, you will need to test more often than what you are currently doing. Esmeralda's numbers could drop quickly once dry is no longer present.
I agree with what Kathy/Kitty said -- BGs in the 200 - 400 range are too high. Above 250 is likely to be above your cat's renal threshold. If your vet's goal is to have Esmeralda surf at around 300, you are, in the long run, risking kidney damage. For the kitties here who are following a tight regulation protocol, dose decreases are earned when numbers drop below 50. The protocol has published veterinary research behind it and it is safe.
I would also encourage you to test a bit more often. It's great that you are routinely getting pre-shot tests. However, Lantus differs from other kinds of insulin -- it's dosing is based on the nadir, or low point, of the cycle. If you don't get spot checks or do a curve, you won't know when the nadir is and you won't know whether to change the dose. At this point, I would be reticent to make any suggestions regarding dose without some mid-cycle spot checks.
Like yourself, I'm a little concerned that your vet may not be entirely conversant with how Lantus is used. Many vets treat Lantus like other, shorter acting types of insulin. It's dosing strategy is entirely different. You could always print out the
Rand/University of Queensland protocol that we use for your vet. If your vet gives you a hard time, then maybe asking here or on the Health Board if anyone has a vet they'd recommend who's in your area may be a good strategy for a second opinion.