Something

bufsize| read     | write    | boost read | boost write | dummy read | dummy write | bufread  | bufwrite
4k     | 666 kB/s | 463 kB/s | 756 kB/s   | 626 kB/s    | 834 kB/s   | 866 kB/s    | 809 kB/s | 847 kB/s
8k     | 699 kB/s | 511 kB/s | 828 kB/s   | 692 kB/s    | 898 kB/s   | 987 kB/s    | 916 kB/s | 960 kB/s
16k    | 738 kB/s | 545 kB/s | 849 kB/s   | 738 kB/s    | 926 kB/s   | 996 kB/s    | 930 kB/s | 959 kB/s
32k    | 756 kB/s | 565 kB/s | 884 kB/s   | 770 kB/s    | 936 kB/s   | 1029 kB/s   | 951 kB/s | 970 kB/s
64k    | 770 kB/s | 577 kB/s | 885 kB/s   | 795 kB/s    | 938 kB/s   | 1047 kB/s   | 934 kB/s | 920 kB/s
128k   | 772 kB/s | 579 kB/s | 894 kB/s   | 796 kB/s    | 952 kB/s   | 1054 kB/s   | 905 kB/s | 819 kB/s

Note that bufread and bufwrite use two buffers, so the effective buffer size is twice as much
Buffersize uses 1024-byte kilobytes, speeds use 1000-byte kilobytes (I blame dd)
Theoretical maximum bandwidth of a full speed USB bus is 1216 kB/s (see USB 2.0 spec 5.8.4 Bulk Transfer Bus Access Constraints)