My modest few hundred hours of actual IFR, and several instrument failures during that time, entitle me to an opinion, but certainly not the last word on the subject. I will leave that to those with much more experience on instruments.
If I were being asked to issue STC approval of an instrument panel layout which did not conform to the design standard, I would be referring the proposed design back to Transport Canada engineering department for review. I agree that a backup AI is great, but if the trade off is loosing the only instrument which actually indicates a turn, the cost seems too big to me.
If the subject aircraft is going to fly enough actual IFR that the affect of instrument failure is really a concern, it should be equipped with redundent systems anyway, then there would already be a back up AI and T&B and the problem has gone away!