The +20 to hit comes from Hunter of Aliens + Hatred. This nets another hit on each attack. That sums up to 5 extra hits, so following your example there would be (10+5)x2 = 30 Hits. That assumes an average roll. An average melee attack beats a full hitting autofiring bolter by miles, counting good circumstances for both. Yeah the devastator could take the extra horde dmg as a squad mode ability (this one didnt), but so could the assault marine with "Wrathful Descent" (just once, i admit, but double the damage and he could just argue he charges another part of the horde. With the right talents he would keep full attacks (he have them btw)) The fat part is what worries us. We cant see Range Combat catching up, and Melee wont lose to single target encounters either. We could revert to the non-errata bolters for 10 Hits on the Heavy, that would net 8 more hits following the example above, but balancing a game by stepping backwards is kind of...awkward.
So what to do? Its not like the play is not functioning, we have a lot of fun. But every time we encounter a horde, we just let the assault marine loose, the devastor is allowed to have some shots too, and thats it. As a GM i encounter the same problem: You either swarm the players massivley, but than characteres like an apothecarius will like bite the dust, or you just have no thread for them at all. I myself nowadays tend to build only ranged hordes in elevated positions with cover. The close-and-personal space is reserved for either ambush or toe-to-toe melee foes. But i'm not nice when im gm'ing, so maybe i dont count