The more I learn about bricks, the less I know. I know that the Whitacre Greer Light Duty bricks are the best domestic option for floors, but matching that in Belgium by looking at brick specs? Yeesh, that's tough. To this day, I don't even fully understand which facet(s) gives the WG bricks the low conductivity that they have.
That being said, there may be a few data points that could help.
Density. More air should translate into lower conductivity. As the density drops, though, you should look at abrasion resistance- and that's not going to be on a data sheet. You'll need to have a brick in hand with a knife to see how sturdy it is.
Alumina. WG seems to contradict this, but, in theory, alumina is about 30 times the conductivity of silica, so lower alumina bricks should have less conductivity.
Color. This is probably not hugely important, but IR is a player in WFO thermodynamics. In theory, a lighter color floor should take longer to pre-heat and allow for slightly hotter ceiling temps, which, in turn, should foster better balance.
Belgium is not that far from Italy. Have you looked into sourcing Biscotto di Sorrento? I haven't known anyone, personally, with a floor that's needed to be replaced, but, from what I understand, these Neapolitan WFO floors eventually fail, so there has to be some kind of distribution network in place for replacement stones. If you can, I'd hook into that.