I just quickly read through the replies so I may have missed it but have you checked the temperature of the pipes at the back of the boiler compared to inside the house? When you turn on a hot faucet in the house, how much does the boiler line out of the 20 plate drop? If it is still hot then I doubt you have a flow problem.
If it isn't too hard to do, I would move one of the connections so supply and return are on opposite sides. Also, if you can put a pump between the other two ports, that would tell you pretty quick if the problem was related to flow inside the OWB.
Does the manufacturer recommend connecting the supply pipe to the top port? If not, I would connect so the return is at the top. You may want to just try it anyway if you can easily change it over. The reason most brands pump this way is to mix the water in the tank. Returning the colder water at the top mixes much better. when pulling out of the top, the water may only be 180 at the very top and cold at the bottom.