Is 640KiB not the whole index? Because partial download would mean having to request new pieces based on the previously-downloaded ones, adding latency to the mix. Assuming just 50ms latency, for 20 packets that would account for a whole second before seeing a result. And 20 decisions seems a lot, though I don't know how the index works.

What bothers me is that you've rounded the time down in your calculation despite ignoring throughput factors like packet overhead, piece message metadata, other protocol messages and processing time. I'd be interested to see some real time measurements. If you decide to do it (maybe for a research paper?), consider using kibis explicitly (and mind capitalization - b vs B).