It is very hard to say because I'm not 100% familiar with the 303SPH footprint (hard to make a square, round). I don't really recommend this panohead for a verity of reasons but still, you can obviously get usable results from it.
You also understand that parallax and stitching errors are not the same thing. You need to have a good spread of control points from the top of the image down to the bottom. 3 to 6 control points is all that is needed per matching pair (more will create problems) but this 3 -6 MUST have a good spread.
It is also very important to make sure the camera is 100% level on the upper arm. A spirit level will confirm this (hotshoe bubble level are ideal for this). When shooting you level up the tripod/panohead for ONLY the first "0" degree starting position. It is of no consequence if the bubble level is slightly out in the other shooting positions so long as when it returns to position 1 (0 degrees maker) it is back to level. Do NOT re-level between shots.
You state you needed +10 degrees tilt to close the Zenith. This seems more then is typically required. This is why I suggest you check for level on your upper arm. Also be sure you have removed the lens cap and lens ring from the Sigma 8mm so that you are getting the biggest FOV (the lens rings pulls off and includes the lens cap).
If you can supply me with a set of images (download) to stitch I could offer more of an opinion.
I realise I may have mentioned things that you already know. But I state them for others who may be reading this thread.