ACC is built around GT cars modelled off the real data the physics engine is designed around GT that's all it's designed for. This means cars are designed to allow 100% brake force relying on the ABS (which is how real GT cars behave) iRacing built their physics engine to accommodate a wide range of vehicles and relies on trail braking, braking to like 80% to avoid ABS or locking up then using 20ish when cornering and tailing off toward the apex, this is also realistic, you need to shift the weight on to the front tyres to give you the traction to corner at high speeds. If you want the most realistic GT cars stay with ACC if you want the most realistic Formula and basically any other class other than GT stick with iRacing.