spring mod could never emulate the feel of a hydraulic brake, knowing how far to push with a spring mod is nearly impossible, and at best inconsistent. and consistancy is key in racing. with a spring you can only make the pedal harder to push by changing springs to harder ones. but then you will limit the pedal travel due to compression limit. and no mater how hard you make your spring it will never feel like a real brake. also with springs, finding the lock point for cars with no abs is a nightmare, and at best all guess work. not enough resistance in either direction to make muscle memory any useful. lets not even go into trail braking.
with hydraulic brakes you get both push and release feel, allowing better modulating of the pedal. unlike a springs, you do not loose pedal travel. and the feeling isnt linear. ( same pressure throughout out the pedal travel) where as in a hydraulic system the more pressure you apply the more pressure is required to go full travel. giving you a much better feel of where you are on your pedal travel . which in turn will provide you with better muscle memory of how far to push the pedals before they lock up. (in long stints this will keep you from going through your tires because you keep looking them up, and keep you out of the pits) easily reduces lap times due to more consistency with brake. not slowing down more than you have to and fast enough to reduce engine lag, and downshifting. with practice. you can tell whether your at 60% brake or 80%. ( with spring at best is 20% or 80% ) and trail braking can be achieve 100% of the time. the difference between staying on the track, or going into a spin because you locked your brake mid turn. brakes usage is just as important as throttle usage. and with proper feel. you lap times with decrease by 2 to 3 seconds guaranteed.
IE professional sim racer dont use springs in their pedals they use hydraulic systems. imaging if we use spring system in real cars. its like a button either completely on/off. because its very difficult to modulate. not very practical to stop a car consistently.
hope this clarified some major advantages of hydraulic system over springs.