I'm lefty and the only ambi part I have on my AR's is the Colt Canada charging handle latch, it just makes it that easier and less awkward to operate the AR with optics mounted. I operate the safety selector with my left thumb, I press the mag release with my right thumb. The ambi mag release is situated really close to the bolt catch and I too often fumble around a bit to find the bolt catch to clear a stoppage or whatever.
I have only one problem with the ejection port being situated on the right side: powder burns. I can't get too close to the rifle while aiming down the sights or I get pretty rude surprises as the gun is firing. I don't have an issue with spent brass hitting me but the powder burns (especially at full auto) are definately unpleasant. Of course its very convinient to have a constant visual on the bolt and know exactly whats going on whenever theres a stoppage; I don't even have to unshoulder my weapon or alter my shooting position to clear jams.
And of course for slings a left handed baseplate sling adapter for my single point or my vickers slings to avoid nasty sling entanglements because the attachment is on the other side of the weapon.
My lefty AR musts:
-Ambi charging handle latch
-Left handed baseplate sling adapter
-Brass deflector
My lefty AR nice to haves:
-Ambi safety selector
My lefty AR will gladly do withouts:
-Ambi mag release
-Ejection port on left side of reciever
All that considered, my muscle memory and preferences have driven me to believe that only 2 or 3 lefty enhancements are necessary on the AR platform, whereas some just clutter the receiver and make it more troublesome to operate other important parts.
my 0.02$
Hans