They don't give the ballast on the Stream 40 but being a fast sailboat it will be a considerable one taking into consideration his big draft and torpedo keel, proportionally considerably more than on the average cruising sailboat, I am sure, that the Stream 40 is a very stiff boat without nobody seating on the rail. Not only because it has a very deep keel and a considerable ballast but because it is a beamy boat.
All those guys on the rail does not mean that the boat needs them there to sail properly, it means that in any boat, including the Opium or the Pogo 12.50, if you sit a large crew on the rail the boat sails with less heel and it is faster.
Narrow boats to be stiff only need to have more RM coming from the keel than beamier boats. The Redline 41 has almost the double of the B/D ratio of an Opium 39 and a considerable bigger draft. The boat will sail with more heel but that's all. I have no doubts that the Redline 41 or the Stream 40 will be more stiff (till 30º) than the Opium 39 even without anybody seating on the rail. Stiffness in a sailboat means power and while the Opium 39 is a very fast performance cruiser this two are not only fast performance cruisers but mostly top racers.
You can have an idea about the power of the boat and its stiffness by the sail area it can fly: the Opium 39 is slightly heavier than the Stream 40 but only can carry a upwind sail area of 63.2m2 while the Stream can carry 90m2 ( both boats with jib) and downwind the Opium 39 can carry 166.3m2 and the Stream 192.0m2.
The Stream is the stiffer and more powerful boat, also the boat with less drag. While racing the crew is there to maximize the boat performance not because the boat cannot sail or cruise perfectly without the crew on the rail.
On races where the 40class racers are raced with a crew you will see them seated on the rail as in any other boat even if the boat has water ballast, a thing that neither the Opium 39 or the Pogo 12.50 have.
In the 40class racers and Open60, the water ballasts are there, not because the boat needs them to sail but to maximize performance, as a crew does on IRC and ORC racers.
The difficulty I had talked about has nothing to do with the stiffness of the boat but with the boat being more "nervous" and in need of constant adjustments to be sailed near 100%. The Stream would not have any problem being used solo or with a short crew if sailed conservatively (80%) and even so it will be with all probability faster than am Opium 39 sailed near 100%. Sure, the Opium would be less nervous and more easy, specially downwind, but that does not means faster