How about give the bikers grav gun and the Tacticals plasma gun? That may be more efficient at the same points cost.
Besides, if you like gravcannon, it may also worth considering putting 2 Grav cannons on
Devs, up them to ten men and give them a rhino. Put the 2 grav cannon and three bolter dudes in Rhino and drive full speed forwards into position 1st turn and start shooting gravs out of the hatches 2nd turn onwards. While other guys with the Lascannon stay in deployment zone for long range firefight. But then, it may need to swap sth for those additional points.