I think the following build is better than grav cannon spam:
4x Centurion incl. Sergeant (340)
3x twin linked lascannon - 60
Grav cannon w. grav-amp - 20
Omniscope - 10
If they are taken as Imperial Fists then they have the Tank Hunter USR.
The new Tyranids will release in November and then Orks first quarter next year - I think grav weapons are going to lose some of their overall effectiveness... Sure they are great now versus Riptides and Wraithknights if you can get them in range. Lascannons on the other hand are always good. I'd just take one grav cannon since you can always fire salvo.
At least magnetize the weapons.