Tame yourself.

Stable Porsche 992 GT3 R Setup for Barcelona - Assetto Corsa Competizione.

Note: Negative toe might still work in the qualifying environment. But all my setups will for now utilize positive or only slightly negative toe as I expect it to be the more sustainable solution.

The Porsche just feels great with a balanced race setup. Ever so slightly willing to rotate on entry, stable in the middle off throttle and again willing to just mildly rotate on exit (apart from high speed, where its pushing a bit). It might be best simulation ACC has to offer. Everything feels really round and smooth, no excessive behavior in any situation.

Overall the new Porsche needs less steering as before and it's easy to turn too much. The high steer ratio helps to be precise.
Be careful on turn in. The tires likely need half a turn or more to build temperature on the surface and they are more snappy on entry when still cold. You can get them warm more equally by reducing camber and going for more positive toe. This will cost lap time in general, but might gain you time if the car is unrealiable on corner entries for you. While the car keeps the crazy rotation off throttle, the center of that rotation seems far more in the center of the car now, than around the tip of the nose, which makes it rotate more like a mid engine car. The rear doesn't feel quite as exposed on corner entry, and you get actually turn into the direction you want to go, instead of controlling a permanent slide. In Q the weight balance is still very far to the rear, hard to do anything about it. I tamed it as much as I could and again no negative toe needed here for (most of the) lap time.

Barcelona
The test track for Formula 1 for a reason: It's variety. Slow, medium and high speed corners challenge car end driver. The fast turns will wear the rears if you are too aggressive and the slow turns need precision and a careful but decisive transition. Late apexes are generally the way to go as traction is key out of every turn. Due to the high speed corners the aero balance needs to be slightly more aggressive than usual, which makes braking zones a little more tricky. Trailing is important to keep the rear in check. Let it dance a bit and settle before commiting to the corners. I highly recommend not going any lower on the TC than 3 - or even 4 in early phases of acceleration to really keep the rear alive for as long as you can in the race. Often you will have a bump just during corner entry and you have to be easy on the brake going over these before you commit to the corner. 

Driving

• Aim for late apexes and dont shy away from a short shift or staying in the higher gear option in the first place
• Double apex is needed in T10 and late apexes in all other tight corners
• your brake is more than a means of deceleration, it also controls pitch and especially in Q this will be key to managing rotation on corner entry - most pronounced going into sector 2 and the medium speed corners there. It generally needs careful trailing to not lock the front tires. And if they unlock you have a sudden regain of grip that sets the rear lose. So be careful :)
• on throttle the car will keep rotating, dont just floor it as that might exaggerate it - take a close look at my throttle traces. TC3 in Q seems fine, but you want TC1 on the exits of T3 and T4, 5 and the hairpin as well as out of the last chicane, and if you can handle it on all other exits, too. TC4 for more stability and predictability at almost no time loss. For the race start with TC3 and you'll like go up to 4 once the rear tires start to drop. 
• dont steer too much, progress into it
• In lap comparison focus on where the car is slow, where its tight and where its wide. Replicate that first and foremost, the other inputs will come automatically with that.

Adjustments

• If too oversteery: Stiffer front spring (this will lead to more oversteer later in the stint, try the other options first), stiffer front roll bar, less rake. One click at a time.
• The brake bias is tricky. If it's too far forward the front will suddenly regain grip and the rear snaps. If too far back the rears might lock. Higher ABS here if you cannot handle it, but likely at a time loss. Clean trailing will pay off.
• higher preload can give more turn in stability, but will come with more nervous exits
• lowering front and adding rear camber might further stabilize the car (in race)
• lowering the caster also gives more predictability
• Some drivers might like a car with less rake for better drivability. It does not seem to be too problematic to lower the wing in turn for 2-3mm lower rear ride height.
• rear tire wear remains an issue. Lower camber all around the car might help paired with more positive rear toe - but its just something you can't fully control. Adding more understeer with higher ARB or a lower rear ride height will be more impactful

Tire pressures
Aim for 27.0 entering the fast corners for solid support from the tire without surprises. You can play with the fronts at lower pressures, so they develop some temperature. Yet we have a lot of high load corners here and a stiff tire feels much more communicative than a soft one. Also i deliberately chose higher rear pressures to keep the temps ever so slightly lower. 

Lap times
Q fuel: 14L (This is safe to use for the sprint races adding the necessary fuel of 40L - maybe add 1 click of rear ride height)
R: 97L (including formation lap)

Race
You need to test how you use the tires. For me this would work for a stint, as I can keep the wear similar (but by far not the same) on front and rear tires. If you use the rears more you will need to add more understeer to the setup with the tips above to keep reasonable balance throughout the stint! Generally you will benefit here in the longrun if you don't push too hard in the beginning, make sure to not overcook the tires in T3 and 4. Rather wait a tad longer before adding power and do so progressively to not force the rear into a slide.

LFM: For the shorter LFM races you'll likely be best advised with the Q set and a bit of fuel, less front pressures, but without any other changes - if anything increase rear ride height by a click.