Actually, all this lenses (including your 110 f/2.0) are modified double-Gauss designs. The unmodified double-Gauss gives very good results with extensions, because it is a symmetrical design and therefore works very well when the distance between the subject and the lens is similar to the distance between the sensor and the lens.
When the lens is modified to become asymmetric, it may give poor corner sharpness with extensions. Between the HC80 and the HC100, the 80 is more symmetric than the 100. It was thus interesting to test whether it would give better results.
The 110 f/2.0 modifies the symmetric design by adding a further element on the front side. It is thus not surprising that it gives poor corner sharpness on extension rings.