Talking to myself and "If only we could be less nit picking". Ahhh.
Didn't know sigma is coming out with a 16-20mmf2.0, that must be something.
Well if budget is not a question for you, have you considered: Leica 24mm f/1.4 Summilux-M Aspherical Manual Focus Lens.
I know that for Old OM mount of Olympus, Olympus had on excellent lens: Zuiko 24mm F/3.5 Shift Lens for OM Mount. Excellent glass. But then not sure if 3.5 is too slow for you and more importantly, if you can actually find a copy of that lens for sale. Also seems your are not happy with your current F3.5 24mm Canon Tilt/Shift.
But then little I know about you, that Rokinon 24mm F1.4 will shine in your capable hands if only you give it a chance. Could you please show and share some images of sharpness problem when wide open with this lens?