It's been a good while, but I'm remembering that Jaheira would leave the party on the Pirates' Island or the Underdark if I leave "too early" to rescue Imoen. Has Beamdog fixed this?
There is never a "too early" to rescue Imoen. Never has been, unless Beamdog messed it up. I've never heard of Jaheira leaving because of Imoen. There might be other reasons she leaves, such as Harper related quests, but never Imoen. Every single time I have played BG2 (not EE admittedly) I've always rushed to Imoen as soon as I have the money from ridding Nalia's stronghold of the trolls quest.