“there is not a “Circle” you can place in Away3D space.”
Actually you can make “circles” many ways, there is not one better than the other. LatheExtrude for instance let you make “circles” with any profile. A ring, a tire, a plate etc.. also % of a rotation help you to build piecharts, halfpipes, like shapes. Use offsets and add more rotations and enjoy springs like results. PathExtrude could also be used but if you make only 360 rotation, Lathe would be better choice. PathDuplicate coupled to some smart shapes would allow to build fans, vents. RegularPolygon let you build a circular plane the easy way. Using some rotational code, combo with LineSegment or Meshhelper (with custom made polygons) give you other options…
As about performance with a “few” of these. Utils such as Merge or Weld or the upcomming 4.1 geometry improvements will certainly give you a hand in that regard. Also keep in mind that even if we work hard on optimalisations, your code is still the keyfactor.