The McLaren was dominating in ACC for a long time. It took until newer generations of cars caught up and eventually passed it in situations. It heavily benefited from the old "setup meta" of putting the cars on the bump stops, because it was already a way to get the rear to stop moving before - I even got confirmation of an engineer on the car, that they indeed put the rear on the bump stop to limit roll. The aero efficiency was always good so even with a bit of rake and max wing the car wouldn't produce a lot of drag. Hence the car was mainly driven like that on almost all tracks apart from Monza and Paul Ricard. Additionally the TC was always near perfect with the engine being responsive in any situation without any noticeable turbo lag and a strong power curve. Now with the update V1.9 tire model, the rear is not that easy to tame anymore. The rear tires are easily overloaded and lose grip as soon as the front end tries to use its potential. This setup is a combination of weakening the front and securing the rear end - and will surely develop further. This is the first functioning version that I feel good enough about to release. There are still a few weaknesses I need to dial out (Noname and the bus stop chicane weight transfer), so be gently in those two corners. 

The Track & Setup Approach
Here on Spa top speed and downforce are equally important - thankfully the McLaren has both and plenty. 270 with max wing isn't too bad! Yet the middle sector is challening and needs stability, eau rouge needs confidence (no worries whatsoever here). My aim was to reduce the pitch sensitivity as much as I could while retaining good rotation through Pouhon (the double left). Also the many transitions in Les Combes and Piff Paff need a responsive rear that doesn't just start sliding. The platform is rather stiff to prevent too much movement, but I'm still allowing some with the bump stops. That might change in the hunt for laptime. The front end has been weakend to not overwhelm the rear. 

Electronics:
TC on 1 or 2 for Q and also for the Race. Maybe 3 to save the tires.
ABS on 3-4 seems predictable enough.
BB on 51.4% seems stable enough for me. You might want to adjust to your liking.

Adjustments (you dont actually need any imo)

• if you generally lack with "feeling" for the car, try adding 5 clicks of pressure or so at minimal time loss, but much better communication from the tire
• also keep the rear tires higher pressured than the front a bit (say 0.,2 to 0.3 - this is based on feeling right now, as it tends to add stability to the car)
• The spring steps are medium sized, front and rear have (almost) the same step size, so adjusting stiffer or softer without much balance change is possible. Try to keep them at the same gap for this setup. I would keep it stiff however - soft didn't seem to work at all. 
• stiffen the front arb if the nose is too pointy. 
• move BB forward if too loose into the corners, yet there's also room to go more to the rear still. 
• Increase front ride height if the ARB das not help you to keep the rear in check and potentially limit front travel further (lower bump stop range, higher rate)

Driving

• the car likes some coasting with tiny throttle adjustments in the corner to correct he position
• use only little steering and don't steer too fast, maybe increase steer ratio even to prevent the rear from stepping out - clean driving is key.
• The car benefits pitching it into the faster corners on the brake a little during initial turn in, this will start the rears rotation and stop it as you go off the brake entirely - hard throttle will keep you going then
• especially out of slow turns feed in the throttle rather than slamming it. Once you are full power beware of the rear snapping - minimal steering allowed
• aim for late apex in hairpins, try to trail deep, immediate but progressive throttle, little to no coasting in tight corners, aggressive throttle in longer corners.
• power curve is flat, shift at 7600 rpm but do not hit the limiter
• be easy with the car during weight transfer, dont ask too much too fast, build into the steering and dont be shy to force understeer a tiny bit

Lap times:
Q: 16L (Not yet happy with the Q lap, didnt put it together, there's certainly a .5 and setup isn't quite there yet Í think.)
LFM: 55L (you can probably use less!) 2:17.2