For me personally 300MB is nothing. I have 180GB included in my mobile plan and 3TB included on my home Internet plan.

For users that do have limits though I think the solution is more trying to optimise clients to reduce the data usage, maybe through caching or avoiding pulling duplicate notes etc. I’m not entirely sure how feasible that is.

If there was some sort of metric for how much data a person produces maybe it could be possible to put users over a certain number of MB in a list and only pull their notes when connected via wifi rather than just not following them.