Let us consider each of these features. Stace is evidenced in 1147, Staci in 1270. Both of those who bore them may have been baptised Eustace . The form with the modern stressing had perhaps coexisted with it for a century or more. Perhaps the stress history of Eustace is similar.
Evidence is scarce.
Now we shall jump to the last syllable, spelt -e. The spelling Eustas occurs in 1275 but it seems to have been uncommon. Of course the -e was still sounded in the time of Chaucer, and let us say that the disappearance took place in the next century. Such a change takes that long. Eustace Families Post Editors' note - Sinclair Eustace of the Irish Robertstown line disclaims being an expert on phonetics and pronunciation but is certainly a very knowledgable student to whom we are indebted.