That's a good question. I assumed that the hashcash benefit in SA had been tuned over the years. I had picked the largest hashcash at the time for the 10-60 seconds per recipient. Not sure what the benefit the GPU can bring is, 10x? 100x?
Yes, botnets or GPUs can be used to accelerate hashcash. But the point of hashcash isn't that it can't be accelerated, it is that it increases the cost of sending each message dramatically. Most spam seems to be sent because it is just so damn cheap to send a million messages. Adding a second to each recipient is a significant cost.
Of course, as we saw with car alarms and immobilizers, there are unintended consequences with things. In cars, we moved from people stealing cars in the middle of the night, to people getting confronted at gunpoint to get the keys. I wonder if increasing the cost of sending spam might just promote the highly targeted methods like catfishing or whatnot.