Michael I did a little more research on fireclay. Apparently, a smooth surface can indicate that the stone is poured into a mold (cast) rather than pressed. Poured clay contains more water, so, when the water evaporates, you end up with more air. More air translates into a lower density and lower conductivity. Poured clay is also very structurally weak.
With the weight of your stone (5 Kg) and the smoothness, I can pretty much guarantee that it's poured. 44% alumina is pretty good as far as firebrick goes, as is the 3 cm thickness, but... neither will make up for lack of thermal mass in the stone.
You seem to be getting great results and appear happy with your current setup, but, should you ever reach the point where you want even more oven spring, then I suggest tracking down a denser stone. Thick, dense and conductive stones are the secret to fast baked pizzas with superb oven spring.
What's your current bake time? Around 6 minutes?