Some of his damage output scales with speed, but ultimately more attack seemed more useful to me, so he's 1 ATK/1 CRIT for me. Substats are mostly ATK and CRIT DMG too, but I think it wouldn't hurt to give him more speed overall instead.
Dust Devil basically turns him into a lawnmower, it's perfect. (Personally I use a maxed out Tonfas artifact to boost damage on the first S3 use. Mostly just because putting Dust Devil on Karin was even more fun.) If you pair him with a unit that increases dual attack chance (is that what it's called?) it's even better since his passive procs on dual attacks too.
I invested MolaGora in his S1 and S3 a bit, enough to reduce turn cooldown for his S3. I'd put some into his S2 if I wasn't starved for them, but as long as you kill something with S1 and S3 you're already golden.
