Gambit Line in the Steinitz Scotch

I've taken to using the Scotch again and on those rare occasions I see a Steinitz Variation (4. ... Qh4), I like to employ a gambit line that Karjakin has used to good effect:

8. Nb5 Kd8 traps the Black King in the centre while White is strong there (8 ... Qxg2 is bad.) It looks like a fairly strong move.

I'm curious about whether castling is a better, worse, or roughly equal option to 8. Nb5. After 8. O-O Nxd4 the position seems pretty passable for Black. White has some compensation for the pawn in the bishop pair, space in the centre, and a small development edge, but that exchange seems to mute a lot of White's attacking chances. Is there something I'm missing in this continuation? (8. ... Nf6 looks pretty weak as an alternative for Black.)

Thoughts?