Ok, after getting to know the lens-camera combo a bit better, here is a next panorama with the Samyang 7,5mm on the Olympus Pen E-P1
click here
I think I made a mistake with the first panorama in my opening post and left the camera image stabilizer on.
I also noticed that this lens is quite sensitive to the Aperture setting.
Yes, with this FOV you need a zenith and Nadir shot, however, with the NN on -15º the Nadir is quite small.
Here is the Nadir cube from a panorama shot with a 6m (20ft) pole from a lens with a comparable FOV, with the R1 at -15º
Karel