I'm speculating, but I'd say quite a bit. If you aren't able to maneuver or throw as well, it's easy to forego even attempting to make additional reads if you are worried about having to make a quick, unorthodox throw. The mental creep, I would imagine, is real.
Take it from Matt Hasselback - the guy who spent 3 years playing on a partially torn labrum in his off-shoulder before ultimately getting surgery as it got worse. He started getting other injuries due to the body compensating for it. He said he tried it - playing with that harness makes it impossible to be accurate on your throws. He said you either play healthy without the harness or get surgery.
I have to imagine simply knowing your accuracy is severely hampered would wear on your own mindset and affect decision-making. Do you attempt to make that throw that you probably can't make, or do you start panicking in the pocket? Blah blah blah, it's speculation but I think warrants consideration when talking about how the injury affects other parts of his game.