@Alex_UTS 

Honestly, if you are getting ~400mbps then that is what I would expect to see, let me explain.  

The 866mbps connection that Windows / Apple / Controller reports is a pipe dream, you will never get anything close to that speed in reality.   Huawei for example (I'm a Huawei engineer) say that the expected speed in real terms is 40 to 50% of what the "connected speed" reads, therefore if you are getting around 400 then its spot on the money.

I have a few 245s here and they get roughly the same result as you are seeing on a 2x2 connection.   

One option would be to try moving to a DFS channel, I find when I move to the 100 - 116 channels i get approx 15% more speed due to the clean airways (~460mbps), that might be an option for you.  Sadly channels 36-48 are now contended in most setups.

To get the full 1350 you would need a card / phone that supports dual connections, basically has 4x radios to allow 2x on 5ghz and 2x on 2.4 GHz.   There are not common and generally very pricey.    The big issue with them is the 2.4ghz frequency, its totally saturated, out of that 450 you are supposed to get, honestly 100 is the max you will see.

I tested one setup with 2x 2x2 wifi cards bonded and the difference I think was 540mbps vs ~460mbps..    yea its faster, but IMHO not worth it due to   cost vs performance

@Alex_UTS 

I agree with @Philbert. In fact, whether it is the speed displayed on the SPEC interface of the product or the speed displayed on your computer, it indicates the link speed, not the actual download speed. Usually the actual download speed of the device is about 50% of the link speed, you can go to Google to search for the relationship between link speed and download speed. If you can get a network speed of more than 400Mbps from EAP, it means that your network is very "healthy".