Steve, I don't think anyone is saying that 1.25" is excessively thick for everyone's use. We just have two members that found the pre-heat too long (for them) in deck oven and high temp grill environments. In a home oven, a 1.25" slab should pre-heat in about an hour. Any good, oven spring producing stone is going to take about that long to pre-heat in a home oven.
Now, could you squeak by with a 1/2" tile? You'd be the first person that I know of to try it. With the density/heat capacity of the material... It's going to perform better than 1/2" cordierite (and 3/4" fibrament), but it might not give you quite enough to hit that magic 4-5 minute NY style baking time realm.
If it were 3/4" I wouldn't skip a beat, but 1/2" might be cutting it close. I'm pretty sure it will perform well, but, until someone buys one, I can't be 100% certain. You will have recovery issues to work around (maybe 10 minutes between pies rather than back to back), but I don't think those will be hard to resolve. If you're oven goes to 550 or higher, I say go for it.
If it did work out, that would be quite the coup for those seeking shorter pre-heat times.