The issue with anything rounded in X-Plane is the number of triangles. If you have a cylinder with, say, 50 sides to simulate roundness, that is 100 triangles. 20 of those would result in an extra 2000 triangles - and that's just in your scenery. Spheres are hell when it comes to rendering, probably upwards of 3000 triangles in one sphere, although I've never even bothered trying to make a sphere for this reason. I usually make use of 6 or 8 sided cylinders when modelling for X-Plane, and I would just altogether avoid spheres. Hope this might have helped. As for memory, every node and vertex requires another line in a 3D file, so add 5000 triangles you've got 5000 more lines, boosting volume. Basically, (and this is just my opinion), rounded things should be reserved for things like aircraft and fuel tanks, that the users will really be focusing on. Make use of texturing to compensate for the crappy appearance of an 8-sided column.