Ted tuned and tested several guns with various STX and Superior barrels with several projectiles. Check it at about 12:45 for the Superior Heavy results.
Like most of the STX barrel liners the Superior Heavy shoots some pellets and some slugs very well. The difference being it shoots a wider variety of pellets and slugs better than the other barrels.
I installed a new superior heavy stx barrel in my Crown 600mm. The JSB KO slugs shoot perfectly and the JSB MarkII 34 gr pellets shoot every bit as good if not better with the same tune.
Strangely I found the .30 HEAVY did not work well with the same slugs that worked really well with the old STX, the .25 HEAVY however seems to shoot anything and everyting brilliantly, go figure!