Over the years, most of mine have grown slower than that, but I'd have to say that you have done a better job with Livingston that what I used to do in the past. I am now a firm believer that hydration, in all its forms, keeps them healthier and makes them grow at a much faster, but still very healthy, rate.
In the past, with the dry routine, my sulcatas would usually be around 3.5-4" at two years old. Daisy was/is a good example of this.
Currently, with the super wet routine, my 10 month olds are over 4" and 400 grams. I'm feeding them the same stuff in the same quantities as I did in the past with one exception. When they hit around 6 months old, I started giving them a few Mazuri pellets a couple of times a week, mixed in with their weeds and grass. They were still growing WAY faster than in the past before the Mazuri though, and the Mazuri really didn't seem to change their growth rate. I'd guess that by two years old they will be close to the same size as Livingstone.
As requested, here are some photos.
These were taken today, after her bath.
Take a look at her shell, where the new growth has started it seems to be thicker, slightly raised, and darker. This started after having moved her to the bigger table, where the humidity is harder to maintain... I am going to be inversting in a humidifer for the room she's in to help boost the humidity.
When I talk about hydration, I'm talking about daily soaks, a shallow water bowl in the enclosure, shell spraying, a humidity producing substrate, and some humidity in the enclosure. I also spray their food with water and some days I'll soak them a second time in the afternoon after sunning all day.