Hi George, I would still be cautious. Names, especially relatively common ones, can be complete coincidences.
And the baptisms you have identified could make that pair a little old for your purposes.
I have a James Hill who died sometime between 1822 (birth of last known child) and 1835 (definite remarriage of his wife), probably in the Plymouth area. There were half a dozen &*%$ James Hills who died in that place in that period, and there is no way whatsoever to know which is mine. It's significant, because the first son's descendant's YDNA indicates that he is not from any Hill family at all, and the two sons born c1820 both had a surname for a middle name that is the surname the descendant's YDNA matches ... so I'm trying to figure out whether the James Hill married to their mother was actually their father. I will simply never know which death was his including whether he died earlier than 1822.
On his marriage in Cornwall, he gave a place of residence in southeast Devon (and the couple settled on the east side of the Tamar in Devon). So again, I have absolutely no way of knowing which of the innumerable James Hills born in Cornwall and Devon circa 1780-1800 were him, and probably never will.
Have you looked for later records in Cornwall of the William Bennett and Elizabeth Dyer baptised in South Hill? For instance, to make sure that one of them didn't die in infancy, or there isn't a later burial record that matches by age at death? What about other children of the same parents, and whether there are records to match them either in the area where they were born or in the area where your William and Elizabeth married?
One thing you might do is contact the online parish clerk for South Hill, who is very knowledgable and very helpful.
http://www.opc-cornwall.org/Par_new/q_s/south_hill.phpTell her I sent you.