GCN can only accomodate 64CU's, custom would be too expensive. Loving the TF numbers people are throwing about. PS5 will not be a massive leap, the pro proved that.
How does the Pro prove anything? To save costs, all they did was double up on the GPU and give it a slightly higher clock, with a few extra features from Vega thrown in. That speaks to nothing except for how they may do a PS5 Pro. A completely different GPU will be just that, different.