Spring stiffness regulates the amount of suspension travel and thus weight transfer. On a FWD car usually the front weights more than the back. An unloaded, dry weight FWD car has usually got a static weight balance of around 60% front - 40% rear. To achieve balance and even suspension travel between the two axles, spring stiffness is usually directly proportional to the supported weight. On a FWD car, therefore, front springs are usually set to be harder than rear ones. In that way, assuming correct suspension geometry and damper settings, 0.4 G in both breaking and acceleration will cause for example the same suspension travel.
What happens when we set the front spring stiffness too low and the rear ones too high? Under braking the front of the car will dive much, causing most of the car weight to transfer on the front wheels, which might possibly overheat (front suspensions might also bottom out and the car's underside might touch the ground, although I'm not sure if this is correctly simulated in GT5). Sure, the rear will now be able to easily slide, since rear wheels might be barely touching the ground (if at all in certain cars). However, the fact that they have to support little to no weight under braking also means that little to no grip is generated. No load means no grip. The total car grip will be therefore reduced, although as a whole it might feel apparently feel "better", oversteery.
You don't want to upset too much the spring balance relatively to the car's weight balance. Small adjustments are ok, but too much will make you lose somewhere. The same principle is valid for everything else, anyway.
I believe it's better to mainly tune the car behavior with wheel alignment, damper settings and stabilizers/anti-roll bars (or the LSD even). GT5 however discourages this as there is no way to find out the weight balance of the car (which by the way also changes with the pilot and fuel weights, or static suspension ride height) other than searching up this value (which might not even be 100% correct) on the internet case by case.
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By the way, here's the exact setup I used for the Focus ST (I posted the last one by memory, there are some slight differences):
- Aero 0/5
- Shift gear @ 6000 rpm max.
- Top Speed 230 Km/h
- LSD 5/15/5
- Ride height -20/-20
- Spring rate 12.0/10.0
- Dampers (extension) 7/7
- Dampers (compression) 5/6
- Anti-roll bar 1/4
- Camber angle (-) 2.5/2.0
- Toe angle 0.00/-0.67
- Brake balance 3/10
) and used SRF to get gold where I managed to pass the leader just before the final tight corner
