I oncet had a conversation with a guy named Baird... a pretty good J/24 sailor in his day. He pretty much confirmed what LakeSailor said... the lowers effect how stiff the mast is. If you have a stiff mast, then backstay does indeed effect headstay sag pretty dramatically.
When it's blowing here on Loch Mich, we'll crank on uppers, and lowers are about the same tension.