I'm also in a similar situation where I have added a considerable amount of weight to my OpenROV. I would imagine to a certain extend, that the mass of your payload wouldn't matter, as long as you make the OpenROV neutrally buoyant. I have not tested my OpenROV in the water with this payload as yet.
I know the OpenROV team has now gone for a different vertical propeller that is more efficient. Something else that I have done is supported the T100 thruster Kickstarter campaign (https://www.kickstarter.com/projects/847...) and will upgrade my thrusters when they become available.
In terms of extra power, I'm also will be installing a second set of batteries in parallel to add additional dive time and compensate for the payload.