All of the slc installation instructions I've read (and there are some new ones out there I've not read up on) require lath on a wooden subfloor. Ardex has a new one that might not. On large format tile, you may want to look into something like the Tuscan Leveling System or the QEP system (cheaper, since you don't have to buy a special tool and available, here at least, in HD). These utilize a t-shaped strap and a break off bracket to hold the tile in plane while the thinset cures, then you break them off (they break off below the tile) and then grout. On any large format tile like that, you want to burn a thin coating of thinset on the backs prior to setting them (use the flat side of the trowel and really push the thinset into the tile). Done right, the thinset gets pushed into the small pores of the tile, and radically inmproves the bond strength. If you don't push it in enough, the thinset can just roll around on the back of a good porcelain. Flat is critical when using large format tile...even a pro gets slowed down when the prep work is marginal on a wavy floor.