To examine the concept of F1-75, a base model was reworked and F1-75-like sidepods were added - let's call them tub-sidepods. Please note, the rest of the model is fairly generic and nothing except the rough idea of how F1-75 sidepods might work should be taken away from these results.
To begin with, let's examin iso and top views. The third picture is from a model without the tub, just a "flat" surface, almost like Aston Martin.
It's clear right away we have several areas of interest - intake, upper surface and rear surface.
The area under the tub-sidepods intake is a high pressure zone, working towards downforce on top of the floor and lift under the intake. No point in trying to figure out how much it helps and hurts overall, unless we had F1-75 geometry and Ferrari CFD setup.
The upper surface is fairly long, probably as long as it can be. On the front, we have some lift generated, as is the usual with the sidepods. In the rear, the tub, we have a higher pressure "pool" which is definitely helpful and one of the benefits Ferrari are looking towards, no doubt. Comparing the results, the overall effect on tub-sidepod comapred to base model is around 100% more lift (only on the sidepod). This is somewhat negated by downforce generated at the front of the floor, as mentioned. The difference is far bigger compared to "aston" sidepods, they are almost completely generating lift all over the top surface.
The rear surface of tub-sidepods works together with diffuser ramp upper surface to form an area of higher pressure. This adds rear downforce directly on the floor, but also increases the pressure on the rear of the tub-sidepod -reducing the drag. Compared to base model, tub-sidepod alone had 50% less drag. This looks a lot like the main reason Ferrari extended the sidepods all the way to the rear - to use this effect and reduce drag. So - F1-75 sidepods are not draggy!
As far as the gills go, they are very likely oriented so they follow the airflow, take a look at flow-viz of simulation. It took 3-4 iterrations to prevent the top leading edge from separating, must be the same for Ferrari and the reason why they raised the intake so much.
I'd like to point out again that the model is very rough and even the sidepod is very far from acutal F1-75.
I wanted to take a look at velocity plots, across the horizontal cross sections. First picture is 0.1m from the ground and the second is 0.3m from the ground.
At 0.1m there is no clear benefit of front tyre wake outwash, as that's still at the floor height. At 0.3m the difference is clear, as well as the difference in airflow towards the rear tyre. At tub-sidepods model, the barge-board-like geometry created a wake going towards rear tyre, and the difference in pressure can also be seen at the first comparison picture. The low-energy wake hits the tyre and pressure-increase is a lot lower. Overall, and take this with a grain of salt, rear tyre drag decreased by more than 10% compared to base model.
In total, compared to base model the tub-sidepod model had around 1-2% more downforce which shouldn't be taken into account as this depends heavily on overall geometry and how all the parts are working together. What's really interesting is the drag benefit - tub-sidepod model had around 11% less drag than base model! According to jjn, the base model isn't too good to compare to in this matter, however I think this difference is significant and should be noted.
Since Ferrari had to extend the sidepods to use the ramp pressure increase to reduce drag, it looks like they made the tub to offset the lift this generates. This is very clear when compared to aston-like sidepods. Afterwards, Ferrari surely used all effects combined with cooling channel design and gills.
