Thanks to Sansal's model importer, there is chance to fix the distortion in the sky above.
I have modelled for the atlantic two whole new sky domes, one for the cloud layer and one for the sky color layer (world sky dome). But contrary to the default SH3 domes, I made the texture mapping coords identically for both and now if I look above, the ugly former distortion is gone
I will test it the next days with other regions and weather conditions. My actual sky domes needs some adjustments for the horizon zone.