I wonder if the car has too much HP to be driveable and you just have to feather the accelerator regardless of the set up.
I grind the dream car Level 24 championship A spec quite alot, it virtually always rains at least once race, usually two and sometimes three races.
I have many different Le Mans cars (Bentley, Sauber, Minolta, Jaguar, Mazda, Audi, Peugeot etc)..
I find short shifting helps me personally, under 150mph keep the revs low but not so low you are just above tickover, but about medium revs, don't get anywhere near the red line below 150mph.
As soon as the car grips in one gear, change up straight away, keep doing so until you hit 150mph, then the aerodynamics kicks in and you can redline it.
Another thing I do is to take alot of corners 1 gear higher than what I would normally take them, so if it's 3rd gear corner, I take it in 4th, 4th gear corner I take it in 5th..
I don't accelerate at the apex of the corner, I wait until I'm through the corner and virtually straight, then start accelerating.
It's not the fastest method, but I hardly ever crash or go off the track and I win ALL the wet races by a very comfortable margin.
Alternatively, buy a Sauber, that thing has got magic tyres, it's a full minute quicker around the Nurburgring than all my other cars in the wet, it's got amazing grip in the wet compared to the rest..

SAUBER