The job of the recoil spring is to return the slide to battery.
Felt recoil is a combination of recoil and main springs.
The slide has to overcome the cocking of the hammer (which absorbs some recoil energy) without having too much residual energy to damage the frame.
If the book says 600 rounds, then why try to go more than that? If your car manual says change the oil every 3000 miles, do you try to stretch that to 5000?
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC
Former President of the ECPT (Eifel Combat Pistol Team)