The following two embedded pages go over the various different Ampak modules. The two chips that are used by Khadas are the AP6212 and AP6255. It appears the theoretical limit for the AP6212 is around 40Mbps and for the AP6255 is around 150Mbps. The best Ampak chip I've seen in an Amlogic box is the AP6356S which is 866Mbps and when I tested it with my router got around 300Mbps sustained via iperf3 next to my router.
However the datasheet claims a different number, not sure why there is a discrephancy though.
The AP6212 datasheet says it is 72.2Mbps instead.
The AP6255 datasheet says that it is 433Mbps instead.
AP6xxx Module Scope-2015-N