I agree that “normal “ road cars are designed for “standard “ suspension.However if you upgrade your suspension without using a LSD You will get the issues that you mention. The LSD has a major effect on the handling which mitigates the suspension problems.Tuning is the “whole “ package rather than just isolated parts.Cars are all different,an E Type has a very long body and wheelbase,while a Porsche is the opposite,just by stiffening or softening the suspension it’s not going to do much for cornering or effective braking.To explain a bit more,when braking for a corner there is a “lag” between the wheels moving into the corner and the car physically moving to take the corner,hence under steer,so using a LSD it kind of removes this lag effect enabling better cornering,also a hard front suspension can make the car squirrel all over the place under heavy braking.So think of it this way,if you upgrade the Engine and you have 700 bhp it’s not going to be very good unless you can stop it effectively so you need to upgrade the brakes.So just upgrading the suspension in isolation is not going make much of an improvement in its self. hope this longwinded explanation helps.For example,using the E Type just tune the suspension and engine etc but keep the differential stock,take it for a spin (and spin it will!), install a LSD and use the setting in my tune for it and you will see the difference. Have a look at the following video and pay attention to the acceleration and braking inputs and how the car reacts to them.
Notice how on certain corners accelerating “pushes” the front so you take the corner tighter and how quick it is to react to steering input,this is the LSD at work.As for the Toe in and Out setting and Camber I am not convinced that the game accurately reproduces changes in an accurate way so I leave them alone.