It should be noted that both THIS IS CINERAMA and WINDJAMMER were given new restorations from the three panel elements and were reissued on Blu ray in 2017 following the earlier Blu-ray releases of both.
Yes, the join lines were very noticeable on How the West Was Won. I also saw the film when it was converted to 35mm and went on general release. It looked appalling - with the joins even more noticeable.