Hello kalivann!
I suggest just as enlargingcloud to drop these Devastators, they are far too expensive, I think.
About the Sternguards, multi-meltas aren't a good option, since they're not relentless. So when you've dropped down, the opponent got a whole round to kill them, before you can use 'em!
So I would pump the squad up to 7-8 man, get a Power Fist, and combi-weapons. If you want melta, throw in 5 combi-meltas.
Vulkan He'stan makes a great combo in this list with the Thunder Hammers and you Speeders.
Also, I would try to fit in some more anti-tank. If you meet Imperial Guard-mech, at 2500 points(!), there will be lots and lots and LOTS of tanks. So like enlargingcloud said, for the rest of the Devastator points, maybe more Speeders or Bikes? For those 190 points left you can(almost) affor 2 x 2 Attack Bikes with Multi-meltas. That's nasty, and not easy to kill, if you turbo-boost with them the first round, and then shoot!
Hope this helps!
/Simon