400 Mbps is "doable" but there are a lot of variables involved to getting there; clear channel (e.g, other devices and WiFis not encroaching on your airspace), channel width, distance, the actual client's capability, etc.)
I'm going to refer you to the Tim Higgins'
review of your router and his testing
methodology.
Yes, he did achieve 400 Mbps but he had a capable client, clear airspace, was six feet away from the router and was using a channel width of 80 MHz. As he moves through his test suite speeds drop and he reports an "average" of 200 Mbps.
Michael, etc. are spot on with their comments and hopefully the links above will offer some insights.
BTW: There is a big difference between wired and wireless. Wired is the gold (well, copper anyway) standard. It eliminates so many variables. You've already used wired to prove that your ISP is really delivering 400 Mbps.
Wireless is shared. Not only do you share bandwidth you share a (sometimes challenging, sometimes hostile) environment; yours and your neighbors'. Sometimes something as simple as a device sitting on the edge of your WiFi area will pull down wireless performance for everyone. Sometimes it's a baby monitor. Tweaking is fun. Beer and cigars help. So might this "
sticky".
It might be fun to have both devices run an Internet speed test at the same time. Each is getting 200 Mbps now. If both get 200 Mbps at the same time well, that's 400 <lol>
