That worked fine... so far madVR 0.87.4 is working very well... I have debanding on, and it has not affected performance.
I was planning on using the new profile functionality to use lesser scaling options when deinterlacing... but in the meantime, I tried using the madVR display rate changer instead of the JRiver one, in order to use IVTC - because most of my interlaced material is network TV shows I record OTA, and they are largely film-based.
So, I "forced film mode" and the result is that madVR deinterlaces using IVTC to 23.9xx fps... and that turns out to be much less GPU-intensive, and also 23.9 fps is easier to achieve than 60 fps, because you have 2.5 times as much time to do the processing.
The result is that with film mode deinterlacing using IVTC to 23.9xx fps, I can use my normal scaling (bicubic75 chroma) and it looks yet better (also due to removing the telecine effect of duplicate frames).
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Update: I downgraded MC19 back to 90 (until the SACD import bug is fixed), and 0.87.4 is still there - so I am guessing that the JRiver install will only upgrade madVR to a higher numbered version.