The original research on using Lantus with cats that established the Tight Regulation Protocol as both safe and effective uses a weight based formula to deterimine the starting dose. Initial dose = 0.25 x cat's ideal weight in kilograms. For an average sized cat with a normal weight (approx. 10 lbs), the starting dose would be approximately 1.0u. The guidelines that were published by the American Animal Hospital Assn recommends a starting dose between 1 - 2u. It's possible that your vet is making a recommendation based on a cat that's not eating an exclusively low carbohydrate diet.
Lantus is a very different insulin than Vetsulin. It is gentler, has a longer duration, and the biggest difference is that it is a depot-type of medication. The depot is what makes it such a good insulin for cats. One other difference is that with Lantus, your dose is based on how low the insulin brings your cat's numbers. If it were me, I would let the vet know you are nervous about starting out at 2u given that when you dosed 2.0u of Vetsulin, Felix's numbers dropped as low as 137. Even with 1.0u, you saw a 144. Given that Lantus is new to you, starting at a lower dose will make you more comfortable.
Also, it will take 5 - 7 days for the insulin depot to form and stabilize. While some cats can have an initial marked response to a change in insulin, it's typical that the response may be a bit less over time as Felix's system adjusts. So, give Felix some time to get used to Lantus. It will also help to remember that any time you make a change in dose, it will take several cycles for the depot to catch up with a change in dose. You may want to read over the sticky notes at the top of the
Lantus board, especially the notes on
dosing methods. There's also a post on the depot. Just so you're aware, Tight Regulation was published in one of the leading veterinary journals several years ago. The Start Low Go Slow method was developed here so your vet may not recognize the name. It's a fairly typical approach to dosing.