Breadman, while I applaud your 4-5 minute baking goal, your desire to get a better stone and the research you've done toward this end, I don't think this stone will perform as well as you expect.
From the photo, it appears that sillimanite is just another word for cordierite. To do successful NY bakes, cordierite generally has to be in the 600+ realm. I'm in the process of looking at a special high alumina kiln shelf that might be able to do 4 minute bakes at 570, but that's only at an inch thick.
You could do some homework and get the density of the stone (by finding out the weight) and contacting the manufacturer to establish alumina content, and that will go a long way in predicting how it will perform, but even in a best case scenario, you won't break a 7 minute bake at 570. I've been talking a bit about gentle oven mods lately, and perhaps if you can squeeze another 50 degrees out of your oven, you might be okay.
Otherwise, I highly recommend 1/2" steel plate. By the time you pay shipping for an American kiln shelf or baking stone, steel will be far less expensive and outperform them all.
