You can stick as much HBM as you want on a card but is it making any difference to performance?
If they could have a 12GB card would it loose 25% of the memory bandwidth, and what would be the performance impact?
If the performance impact was minimal, dropped the price sub £600 and reduced power/thermals that would seem a good trade off.
That HBM2 isn't cheap:
https://www.fudzilla.com/news/graphics/48019-radeon-vii-16gb-hbm-2-memory-cost-around-320
The HBM2 was already mounted when they test the cards and oick the failures for Radeon 7.
Not possible to remove a stack once mounted to the interposer. So you have a 16GB card or it goes in the bin.