A lower and upper arms relate to camber control and really, there's no problem with using the halfshaft as the upper control member as long as the diff is tight and the stub axle has no excessive play.
Front toe out is great for handling but crap on the roads. A good example is a 3rd gen camaro or firebird, those are set up for toe out and they are awesome in the corners but on uneven roads the car will pull like crazy. Never toe out the rear, it will rear oversteer, snap oversteer. Problem is, that's exactly what our trailing arm suspensions do if the halfshafts are level, anything away from there, either up or down promotes to toe out meaning you have to set more initial toe in. Best to have the shafts a little under level swinging to or slightly over level but not further over than about the same angle it's under statically + the tolerance you set in initial toe in so that the arm doesn't swing further and goes to toe out again.
There's another problem, especially with the early C4 IRS, they have a high roll center, great for roll stability but theirs is a little on the high side, causing a jacking effect. There are plenty of guys who have experienced snap oversteer on a C4 because of it. I remember this video of a guy in some twistes who suddenly lost contol