Async or sync methods in webapis?

See more:

AsyncTask

WebAPI2

Should I use Async or simply Sync methods in my WebApi aplication?

I use mvc5 and EF 6.

The operations are mainly CRUDs and listing ( select against the database )
and the DB queries are running quite fast.

The system is used by about 1000 concurrents users.

Do I gain performance using Async?
What tool can I use to evaluate both scenarios ? ( stress test )

Thanks in advance!!

What I have tried:

I'm using EF, generic repository, UOW, all with sync methods and I was asked to decide if to pass to Async..