Here I'm using the 441,66 studio drivers. With them, I can render your scene in an 8GB GPU even without enabling the out of core feature.
About your problem with the cracks seams when the motion blur is enabled comes from some kind of issue with the motion "v" velocity point attribute. By default, the plugin computes the deformations motion blur using it, but if it is not available, it tries to compute the motion blur from the mesh deformations directly, something that in your case is possible because the objects have a constant topology. Can you please delete this attribute, as you can see in the attached image? The problems seems resolved when the plugin doesn't use the velocity attribute to compute the motion blur.
Computer Graphics Software Developer
Octane plugin development and support: LightWave and Houdini