IIRC, stouter driveshaft and half shafts, better connecting rods that are actually forged not sintered, and MAYBE heads from T56 aluminum heat treated to T6 (or maybe all Hellcats have that feature) This just off the top of my head, as I do remember kind of envying the list of modifications that, to me, were not just marketing fluff but actual engineering improvements.
I don't think it ends at "slap a 2.7L IHI blower in it to replace the Hellcat's 2.3" by any means. They (Dodge) don't want warranty grenades.