Related question: Does SBT use the Fast Scala Compiler (fsc)?
Can fsc be used with sbt; would it be practical; and if so, how to integrate fsc into sbt?
fsc cannot be used with sbt.
https://github.com/sbt/sbt/wiki/Client-server-split shows that the sbt team is currently working in a similar/related direction. They're turning sbt itself into a local compile server — like fsc but better. I wouldn't be surprised if once they're done, fsc is deprecated and removed from Scala. (fsc has never worked all that well or been very actively maintained.)