The PPS is not an especially large sidearm. The weight of the spring needs to be correctly matched to the power of the load, otherwise you may risk timing errors or excessive wear to the pistol. I would trust that Walther has correctly sized the spring from the factory, and resist the temptation to experiment. Remember you are holding 400 to 600 joules of energy in your hand. I like my fingers.
Have you tried different techniques to rack the slide? The slingshot method is popular, but requires a fair amount of strength to pinch the slide between the thumb and index finger. Instead, try an overhand release. Racking the slide ought to be about technique, not strength.