Point for point, Shoota Boyz are more accurate than Marines.

I would consider myself relatively savvy when it comes to list optimisation, and I am of the opinion that large Nob squads (as troops; they have both the CC potency and durability that makes them useful), Lootas, and Deff Rollas are fairly integral to an Ork battle plan, as well as a large Shoota Boy squad for holding mid-field objectives.
Whilst not top tier, I believe that are still quite competitive.