Dimensius was also just an avatar of said Void Lord, and according to "Chronicle Vol. 2" was only a fraction of the Void Lord's actual power - a temporary manifestation that nonetheless threatened two worlds (and destroyed one). As I said above, we don't really know how powerful the Void Lords are relative to a Titan, and the Titans themselves also didn't know much about the Void. I would imagine that you are in right in that the co-mingled power of a Titan and Void Lord in the person of a Dark Titan would be greater than either, but that still doesn't tell us much about how an individual Void Lord might compare to a realized Titan.