Do the adapters have a PC app that reports performance? I have some TP-Link 500Mb adapters, and another TP-Link 200Mb Homeplug wifi extender. The 500Mb devices report a connection speed of between 100-150Mb/s, but surprisingly, the 200Mb adapter often shows about 170Mb/s! Work that one out.
The download speed from Steam does not reflect the true performance of the adapter. You'd need to run some network performance tests to another device on your LAN, or even try copying a large file in Windows. It won't be totally accurate, but it will give you an idea.