Do you torte and fill cakes baked in the topsy turvy pans or just frost and stack as is?

AI've never made one but I would suggest you torte and fill as usual. Otherwise you will be serving some large chunks of just cake, with no icing/filling in!

When I make topsy turvy cakes, I always have several layers of filling (for all my cakes for that matter).  I think the fillings are the best part, and visually it is so pretty to cut into a multilayered cake.  I have used the mad dadder pans a number of times, and use them for the top one or two layers of the tier.  For smaller diameter cakes, I fill the pan more fully and tort to get the top two layers.  For larger diameters I am more concerned about baking evenly, and  fill it enough so that the thinnest side will be a little taller than the desired height for each level of cake in the tier (ie - makes just the top layer and the extra is to allow for levelling).  I also use heat cores and wrap with baking strips.    I then bake rounds for the lower levels, which can be tapering to a smaller footprint as desired, and as many as needed for the height desired. Then level and stack all the cakes up, and carve the sides to the smooth tapered contour. Then separate, fill, do a little touch up trimming if needed.  If freezing for a while, separate, wrap each, label which level it was(!) and freeze until ready to assemble.  I am sure there are many ways, but that works for me.  HTH