Scala is a great language with terrible (in comparison) tooling. I'd use it (or F#) over C#, if IDE support was not as slow as it is, and if it was comparable to ReSharper/IDEA Java feature-wise for refactoring.

How so? .NET is 2-4x faster than Scala in benchmarks (https://www.techempower.com/benchmarks/#section=data-r20&hw=...). Do you just prefer the syntax and language features?

F# is probably a more relevant comparison - it is kind of the Scala of .net. But in any case C# is much more widely used than F#.