Ah, I get it now, sorry for missing the point.
Maybe I’ve had similar issues, although I’m not really a keyboard player so I may have mistaken it for bad timing on my part 
I will have a good look at the scenario again on my own setup. Although I don’t really seem to have very obvious issues playing in, I use the midi in on the 18i20, and audio doesn’t seem problematic either. The artist I am working with sings and plays guitars via the 18i20, and doesn’t notice any issues.
The one thing that is a big surprise for me, is latency, or lack of it. I don’t know if different interfaces work better or not with Cubasis, or if there is any sort of auto delay compensation. Even when playing things and adding effects during the recording, there is no perceived latency with the live signal and what Cubasis is processing/adding live.
Again, these things may be there and so small that I’m simply not noticing them. But in my experience, and as far as latency or any kind of buffer issues are concerned. Cubasis and the 18i20 is easier to set up, as in no setting up, than previous RME/Nuendo/Cubase Settups.
Cheers,
Dave.