With the latest patch, a warlock has lost a bit of damage too.

They used to go Immolate + Conflagrate = dead trash, with others casting too.
Now that Conflagrate has lost some of its damage (has become a DoT component, instead of all at once), and the mob is dead before the dot ticks, they don't deal as much DPS on trash.

Not that trash DPS really matters, as you're killing them all extremely quickly.

For a boomkin, if you manage Eclipse accurately, you should have comparable dps to shammies. Take a look at WWS or wowraid reports. The boomkins and elementals, do comparable dps at similar levels of gear. It's just the elementals spam a single button to do it, while the boomkin needs to manage eclipse for similar dps.

For trash dps, which doesn't really count, it will depend on the mob; very low health mobs, will die to a Wrath hit before the Lightning Bolt hits, which will show much higher boomkin dps then shammy. But if LB kills trash before the 2nd Wrath, the DPS will be higher for the shammies.

I think the boomkin brings more to the team, but the warlock does not require anything other then mindless one button (click) spam, which makes them a superior choice for most teams.