Actually, you'll only see Hordes being taken out in one round (by one opponent) if they're fairly small, roughly the 20-30 magnitude range. At best, a starting Dev with Heavy Bolter and Metal Storm rounds who lands all six shots will do 13 + 1d5 (6 shots + 6 damage from MS rounds + 1 from exploside damage + 1d5 from UD). That's still very good, but that's a perfect shot. The damage will increase a bit (even more if you use SoI as doubling the entire magnitude damage and not the base).
Magnitude damage is calculated in two steps, that is something you should be aware of
1. Calculating # of hits. (In this case: 12 + 1 + 1d5)
2. Translate hits into mag damage (With SoI this becomes: 3x(13+1d5) mag damage)
One-shotting a Horde of more than magnitude 60 will take some doing, which seems reasonable. The downside to this is that the Horde-killing ability of everyone else went down, too; just not as much as the Dev's ability. What really hurts is the weakening of the frag grenade, since that's what a lot of the other specialties use for Horde damage.
You only need to learn as a GM to scale down horde mag sizes comparably.
Actually, the Dev as-is is really more of a sniper if you look at the Talents available. IW and UD specify heavy weapons, but the majority of Talents are best if used with a single-shot type weapon since they help with called shots, shooting at extreme ranges, firing and moving, etc. If the Dev was to be a real heavy weapons specialist, there would be more heavy-weapons-specific abilities they could tap. The specialty is really just a catch-all for "guys that like to shoot."
Storm of Iron will make the Dev the anti-horde guy again. Also you are being too pessimistic: KTs will tend to assign roles among themselves. SO the team will need to face vehicles and concludes they will need two multi-meltas. Who will get them? Most likely th Devastator of the kill-team will not get empty-handed.
Devs have still very cheap BS advances. They get Storm of Iron. They get Mighty Shot on first rank. They either get a nice horde bonus or another to-hit bonus (although they don't really need it that much at higger ranks).
DW is still very much a shootists game. No, I don't feel bad about the Devastator I play. Okay, right now he is brandishing a Lascannon too but that's another story.
Alex
Good catch though.