Sorry, but most of that exhaust gas thread was filled with conjecture about increasing and accelerating mass flow with little consideration of the practicalities or the specifics. Simply saying that mass flow is increased because you are directing exhaust gases under the floor doesn't tell us anything.
Once the nonsense had died down in that thread the major practical stumbling block of blowing hot exhaust gas under the floor was pointed out - decreasing dynamic pressure and increasing static pressure (must make sure I get that the right way round!) in any way, as per Mr. Bernoulli's guidance, is obviously just not desirable. There is still a pretty large volume of hot gas going under there that will have an appreciably negative effect regardless of how quickly that gas cools and regardless of what other hypothetical benefits it might create somewhere.
Using hot gas forward of the floor or around it I can understand because the hot gas is not having any negative effect on the floor itself anywhere, but putting it under the floor? I cannot see at all that this is what Renault are trying to do and you certainly can't get that just from the direction their exhausts that you think you can see.