Here's something to think about, the datasheet quoted says that film speed can be doubled by increasing the first developer time by 3 mins, could the same be said if you reduced its film speed down to ISO 100 for example and reduced the first developer time? Or is that not possible?
I have read that shooting any film at a lower film speed can give you finer grain, but i guess you would have to compensate with development in some way or else you would have an under exposed image?