I see
Kasia and
Kassia as totally different names with different origins.
Kasia is a short form of the Polish
Katarzyna and
Kassia is the default Greek spelling for
Cassia (Latin spelling). I believe
Kassia was an ancient Greek poet. For the record, I like both
Kasia and
Kassia.
Re: the pronunciation of
Kasia, I do say KAH-sha, but I think you could get away with KASS-ee-a which is how I pronounce
Kassia. I think if you're going with KASS-ee-a, perhaps you should consider the
Kassia spelling.