I too grew up believing that I was of Welsh descent. But then, family legends are often truer than life. As far as the suggestions that all who spell the name "Hewes" are related. . .this is not true. There are different lines that adopted the spelling, but are not neccesarily related.
The information that you speak of does not originate from Mormon sources, but comes from a book written by Joy L. Hughes-Jacoby, entitled "The English Ancestral Family and Descendants of William and Deborah Hewes of Ouldman's Creek Plantation, Salem County, New Jersey and of Marcus Hook, Chester County, Pennsylvania". The book is no longer in print, but is published by Heritage Press, 2000. You may be able to get a copy via interlibrary loan.
Apparently, someone took information from the book, and submitted it to the LDS Church.
While certain lines in the book are controversial, I do find credibility in the author's english research, which connects the Hewes line with the Huish (Hewyshe)and ultimately back to William de la Heuse and even further back to Rognwald, County of Maerc, Normandy.
One suggestion concerning the Welsh connection is that the ancestral home of Somerset is just across the water from Wales, and the Hewes family had frequent business dealings with others in Wales. It is also possible that there could have been some intermarriage with persons of Welsh descent. The line itself, however, does appear to be English.