I tried the obvious headings to test the doppler effect worst case. Could not see anything on either the signal strength or the actual data rate during a large file download (1 meg file).
185kt GS is easy. I've done 210kt once. Vne is 187kt
indicated which is easy to reach at say 75% power and in an approx -1000fpm descent, and with some tailwind anything is possible, but this is cheating of course
... 80% power and -500fpm gives ~ 170kt IAS at 5000ft. I am writing up some notes.