So expected maximal TCP/IPv4 goodput for your configured rates would be approximately (since I need to guess your per-packet-overhead setting, I assume true ethernet with a VLAN tag, to be on the safe side):
900 * ((1500-20-20)/(1500+38+4)) = 852.140077821
800 * ((1500-20-20)/(1500+38+4)) = 757.457846952
200 * ((1500-20-20)/(1500+38+4)) = 189.364461738
Which is close to what you actually measure, so it seems this router shapes ~1500 byte packets at gigabit rates. It would be interesting to repeat the test with smaller packet/MSS/MTU size to see how the router performes with considerably higher packet-rates with smaller packets.