eta-- Almost anything's possible when it comes to personal names, family history, etc. Took a look at an English surname dictionary and there's no Grow in there, but there is Grew and Grewcock (li'l Grew) from AN la grue 'crane'. Thing about the 'w' in Welsh is, I'm not sure it was used all that often in Middle Welsh (which was ending around the time of your ancestor's emigration). Good luck.