WR1 Bro wrote: I believe on a normal STi they are plastic.
Incorrect. Mine are exactly the same as you show on your pic. Maybe earlier STi's or WRX's run on plastic.
I assume they use(d) plastic to cushion a bit the vibrations that could be passed on by the anti-roll bar. Downside is obviously that it will take some sharpness out of the handling.
Assume you want to replace them with good steel ones. Wouldn't go for any lightweight stuff as lighter metals tend to allow for more flex.