I have no concrete evidence that James Kayess was the James Keys baptised in 1788 at Claines. However, there is much circumstantial evidence to support the hypothesis:

• There does not seem to be any trace of James Keys in the Worcestershire area after his baptism (unless the burial of a James Keys in 1802 relates to this James Keys rather than his father or grandfather).
• His age of 50 in the 1841 census is consistent with a birth around 1788 bearing in mind that ages were rounded down to multiples of 5 years.
• Most of James Keys' siblings (as well as their mother) changed their surname, some of them to Keyess.
• All other Kayess births, marriages & deaths before 1850 on Ancestry, Find My Past & Family Search appear to be descendants of James Kayess or their wives (except for the death of Daniel Kayess in 1591!) so this makes it very likely that there has been a change of name.
• James and his sister, Esther, may have come to London together. Esther married in 1817 at St Marylebone and James married in 1821 at St George, Hanover Square; these two churches are less than a mile apart.
• At the time of the 1841 census both James Kayess and Esther were living at Stratford (albeit about 1½ miles apart).
• The friend of a third cousin of mine is helping him with his family tree and I am grateful to her for drawing my attention to the possibility that James Kayess was James Keys and to the connection with the West Ham Abbey silk printing works which I had not discovered. In 1821 James married Elizabeth Tucker and at the time of the 1841 census the West Ham Abbey silk printing works was owned by Elizabeth's brother, John; James was described as a silk printer. By 1851, James Kayess' son, another James, was Manager of the printing works. In 1841, Esther's son James Maple was a silk dresser, some of the Crutchfield family (almost certainly relations of James Maple's wife) were block cutters and these presumably worked at the West Ham Abbey printing works. Elizabeth's sister, Mary, married John Baker in 1821 and he held a senior position at the works up to 1832.