Thanks for the suggestions everyone.

I wonder when Autodesk will either add a GetBoundingBox method for splines, or fix the method for blocks which contain splines. It would seem to me that the top and bottommost points could be determined by looking either at "corner" or "end" points, or along the curve for the real zeros of the first derivative of the y(t) function of the curve falling between (0,1). Then take the minimum and maximum y values of this set. The left and right most points could be determined by looking at the corner and end points and also along the curve for the real zeroes of the first derivative of the x(t) portion of the bezier function to create the set for the possible max and min x points. I have played around with beziers in Illustrator and have found variations on this method useful to find tangents to curves. Since all bezier splines are mathematical functions, it is possible to find the boundingbox with arbitrary precision.

I have not tried to code up a routine for this in autolisp as the method I used above works reasonably. However, I would think that the technique described in this post should have performance advantages over the method employing GetClosestPointTo.