Here is my take on this:

1-Of your two audio interface choices I would get the Focusrite 4i4. A lot of forum members use Focusrite interfaces so you would get more help, if needed, using one. You will have 4 line inputs so you can have your RD-800 in stereo mode with two other line inputs still available. Plus you will also have two inputs in the front of the unit for mics, guitars, etc. You can have your RD-800 MIDI set up as you do now along with the above audio outputs.

2-No, no audio interface will improve BiaB MIDI sounds. This is an another whole subject but to improve BiaB MIDI sounds you will need better sounds sources. A rule of thumb is the more a MIDI sound source will cost the better the sound will be, period. We can talk about that later if need be.

3-BUT to improve RT sounds you will definitely have to replace your computer speakers with near field monitors. Near field monitors can be expensive. But even a pair of Presonus Eris E3.5-3.5" Near Field Studio Monitor (Pair) (E3.5) for $99 USD would sound a lot better than those computer speakers:

https://www.amazon.com/PreSonus-Eris-E3-...0665&sr=8-6

I want my last spoken words to be "I hid a million dollars under the........................"

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware