Never mind-- figured it out.
In ‘C:\Program Files (x86)\Sonic Pi\app\server\ruby\lib\sonicpi\scsynthexternal.rb’, I added the ‘-H’ switch to the ‘boot_and_wait’ statement in the ‘boot_server_windows’ function, passing it the name of my soundcard (in my case, ‘Focusrite USB ASIO’).
Now I’m getting virtually no latency when using audio in!
Bryan