That wouldn't be a circle though
Depends on the Tilt values but yes it will more likely result in an ellipsoidal form. Doesn't bother me though as I have it spinning round my proscenium arch.
so you couldn't apply it around an existing position
yes and no. If you set the values to the absolute maximum you will ever use running it at 50% size will result in a weird but beatiful relation to the existing position. However if you choose to use a backdrop position this effect will not work as wanted. Again this is a matter of playing around and finding out what works best.
it is much harder
the best things are rarely the easiest
Again a reason to finally implement relative values in key frame shapes. Together with release frames (similar to cue release for chases) this would finish the ley frame feature and we could kill the generic shape generator (including the shapes as presets for KFS). The shape generator has done an outstanding job over the years but is hopelessly outdated by now...