I’ve found that neither adding spices (I make my own secret recipe spice) in the bag, or right before searing work well. I get a better flavor barbecuing with a light coating of olive oil and secret spice rub on one side. Plus, you get the added smoke flavor. I have the barbecue action down though and never dry out or burn my steaks.
I think sous vide may work better on beef that has been marinated before, like some tri-tips or something.