You design it to hold more data if it can't process it fast enough, sure having 256mb will affect it, obviously, but 4GB of ram, running 1.7x+ faster than GDDR5 ram is as fu8ture proof as it gets.
I'd rather get 4GB HBM than 8GB GDDR5 at 6gbps. maybe if the 8GB was running at least 8gbps, but...