I came across that spelling in a text on the Cornwall Subsidies for King Henry VII, in Constantine Parish, 1524, a Robert Syse was listed.
I am of the opinion that the spelling was a variation of the name Sye created by the local tax collector to help him in keeping his records straight. Also listed in that parish were John Sy, James Sys, and of course Robert Syse. Changes in spellings of other names in the same parish occurred even when they lived in the same home and when they were "juniors".
I have been looking into the names Sy and Sye, Syse is one I have not seen other than the above example.
Richard Sye
rsye@fnbop.com