For what it's worth, I got in touch with Tim Steele about this and he said:
Certainly worth experimenting with, but it might not be a panacea. On the other hand, every prebiotic supposedly feeds a different species of bacteria, so there should be some difference. And it's also worth noting that people anecdotally claim that arabinogalactan seems to noticeably enhance the immune response. So, n=1 accordingly.
Tim Steele said:
I've looked into Larch AG pretty hard. It's just another prebiotic and does the same things as potato starch, inulin, banana flour, etc... In the studies, they have to use 20-40g to get the results, but the supplement pills are like 2-3g and expensive.