edit:
Ok, as I mentioned earlier, you shouldn't need to check normals in the export dialog. In fact, forget about the Normals modifier. You should making proper use of smoothing groups, AND (this is the important part), omit normals in the export dialog. This is what I use:
This operation seems to omit normals in the export operation, and Octane rebuilds normals based on smoothing groups. (It also takes half the time to export this way)
And, BEFORE importing, get into Octane preferences and check this:
This last step may not be necessary, but I check it either way