Not sure why I'm bothering but I'm new here so I have to start somewhere. I promise I'm not anal...just thorough(!)
Siobhan is the Gaelic form of Joan. Judith and Julia are variants. Gaelic is a language in its own right and therefore has no link to your proposed phoenetic spellings in English.
The marvel of the internet...
http://www.ireland-information.com/heraldichall/irishgirlsnames.htmAn interesting one is "Menzies" pronounced "Ming". I can't remember going down to the "John Ming" newsagents and getting 20 Embassy, so I guess both pronouciations are valid.
Your spelling is interesting because the "Ch" being pronouced as "Sh" dirives from the influence of French. So in itself, your example is self-damning. I admit this is often difficult to avoid!
Oh...I have a BC-Rich Warlock NJ Series (circa 1997) fitted with a very special Warpig on the bridge and a DiMarzio Super Distortion on the Neck. I use a POD to a Power-Amp into a Marshall 4 x12 cab. I also have a Fender Jazzmaster '62 re-issue in the classic tobacco suburst finish, with no modifications.