You either believe the testing and the manufacturer that Kerdi, properly installed, is waterproof, or you don't. Or, you don't trust your ability to do it right. If it is waterproof, as long as what's behind it is stable and is recognized to be able to hold and bond with thinset for a good seal, it's good (there are millions of tile installed on drywall outside of a shower, so drywall is known to hold tile and thinset well as it will hold onto Kerdi). If you don't believe this, don't use it! If you don't have confidence in the manufacturer, don't use it. If you don't trust yourself that you can do it, don't do it! Versabond is a modified thinset (lightly modified, but still modified). There's nothing wrong with using cbu, or adding other products that are known to be compatible on top of it to give you extra margin except for the costs (and starting with something else may work as well and be less if you're going to combine products). If you feel that extra margin is worthwhile, go for it, it's your house, may make you sleep better. Using a modified thinset voids your warranty, as Schluter specifies only the use of an unmodified when using this material. A worst case test will be the flood test. A shower does not normally have standing water in it, and if it does because of a bad drain or something similar, it typically doesn't stay there for the common 24-hours or more a flood test checks. Very little moisture normally gets below the tile in a normal shower, but it still needs to be managed, and if not, can accumulate and cause problems. With the slope on the pan, there should be no pooling of any possible moisture underneath the tile on top of the membrane. The sheet material is waterproof, a proper 2" minimum seam may wick a little water, but not beyond the full width, unless you have too much thinset in there (this is where the technique is important). But, even then, if you have the proper slope to things, there is no hydrostatic pressure to push water into that seam - the only time it would happen is during the flood test.