Not necessarily, if it fails to negotiate an optimal setup with the Marvell Avastar chipset/driver it might still be a firmware issue even if it negotiates better settings with other devices. your assumption that all devices operate the same is incorrect.If there is a limitation you should see it in all devices not just the SP3.
If it hasn't been done it needs to be done.
But there are a whole raft of other issues as well... just the tip of the iceberg
This Is Why Your Wi-Fi Is Always Slow
10 reasons your Wi-Fi speed stinks (and what you can do about it)
It could be your other devices are causing the problem.
The surest way to never fix a problem is to do nothing.