I'd ask people that know you and have experience in the field for a realistic opinion (maybe a professor/assistant/phd?). It does definitely not hurt to code all kinds of systems stuff. Try to tinker with some low-level optimization problems. You should be an excellent coder too. In our case it does help to know the JVM ecosystem, including HotSpot details.

And (shameless self plug), implement a Truffle language. Start here: https://www.youtube.com/watch?v=pksRrON5XfU&list=PLN193mR3Js...