Depends on your height. The visibility is perfect for me, I'm 5'9. I dismissed anybody that criticised the driving position as being too high as talking absolute rubbish and accused them of just saying it to follow the trend of what everybody else was saying. That was until my brother in law drove my car and the mirror was clearly obstructing his view, even with the seat in it's lowest position.
We have established in the Yaris groups that if you're 5'10 or under, the driving position is amazing and the mirror is fine, no different from any other car. If you're over 6 foot, you are probably going to dislike it, You can either lower the seats using aftermarket subframes, or you can modify the mirror to sit it higher.
With regards to the view out the rear-view mirror, its a very small window and you would struggle to reverse park without the rear camera. The rear camera is amazing though, very easy to use and makes reverse parking easier than any car I have ever driven before.
However, I don't think that any of these things are an issue in the GR Corolla. From the videos I've seen online, the driving position is much lower and the mirror should not cause an issue for anybody, regardless of height, also the back window is larger. Please remember that the Yaris is a homologated rally car, rally cars are designed to have a higher seating position because it's designed so that the driver and co-driver can easily scan the road ahead. The Corolla is developed as a road car only, so I'm confident that you will not get any of the driving position, mirror, rear view issues that people experience in the Yaris.
This was a video I made when I first got the car, I do talk about and try to demonstrate driving position, but at the time, I couldn't comprehend how different it would be for a taller person.