A couple of things: cbu does not increase the deflection rating of a subfloor - IOW, adding some will not make the floor stiffer; 5/8" material, is the absolute minimum strength required to lay any tile. If that first layer is not T&G, I'd want a second layer of ply before I did anything. Then, the 'system' requirement would be that the floor deflection ratings should be adequate for setting tile and that would depend on the type of tile, ceramic or natural stone. It's been awhile since I read the installation instructions on the Kerdi pan, but I seem to remember they want the floor structure to be at least L/360 deflection rating which is the minimum for ceramic tile. This is determined by the thickness, depth, spacing, species, and length of the unsupported span of the joists. If your deflection ratings are at least L/360, you should be able to just thinset down the pan and go. If you were going to tile the rest of the room, I'd consider adding another sheet of ply, at least 3/8" thick (the thinnest suitable to a second layer with tile) under the whole floor. Hassle with 5/8" first layer, especially with OSB, is that it's very easy to strip the hole with the screw. It may take predrilling the holes and careful torqueing the screws to get things together well.