No, I haven't seen Pospisil here. If he's fit, it's an insult if he wasn't selected after the way he played in Spain. I'd have him on a plane and dump FAA, or at the very least play him in the doubles.
Shapo had a good, tough win against #6 Tsitsipas, flogged a hapless #7 Zverev and a narrow loss to #18 de Minaur who just goated in front of a massive Australian cheer squad. Djokovic was taken to two breakers by #147 Kevin Anderson, flogged an injured #9 Monfils and beat #33 Garin less convincingly than Anderson did.
By any sensible measure, Shapo has had tougher matches and I'd certainly rate him a chance based on seeing both of them.
Doubles has been a lottery. Troicki is an excellent doubles player, but he's the only one on the team who is. I think Shapo and Pospisil could definitely take the doubles. FAA has been shaky in singles and really quite poor in the doubles. Again, you guys really stand a better chance with Pospisil.