A very interesting report and intense pressures used and reported, 50 Gpa, more than 493,000 earth standard atmospheres for the pressue. The exoplanet 55 Cnc e is cited as a possible diamond exoplanet in the report, http://exoplanet.eu/catalog/55_cnc_e/
With more than 4300 exoplanets now, I do not know if any are known for sure to be diamond exoplanets. The paper cited concludes.
[5. Conclusion Combined with the existing experiments at low pressures, our new experiments at high pressures show that water can convert silicon carbide to silica and diamond. With our finding that carbide planets will readily convert to silicate planets in the presence of water, the number of carbide planets in existence may be even lower than current predictions. Furthermore, a carbide planet could convert to a type of planet that, to our knowledge, has never been considered before: a planet rich in both diamond and silicates. The unique mineralogy of the converted carbon-rich planets would make the planets un-Earth-like...]
It will be interesting to see if exoplanets like this study models are confirmed.