TOO MUCH VALVE SPRING

Two drawbacks to running too much spring pressure. First, as others have mentioned is wear and load on the entire valvetrain. Keep in mind that the cam sees the open pressure times the rocker ratio. So 300# open pressure is 450# at the cam lifter interface with a 1.5 rocker, placing greater load on the oil film (insert zinc debate here).

The next is a bit more subtle, but still important especially on a street engine. It simply takes more energy to compress a stiffer spring. This is $4/gal energy that isn't turning the driveshaft. Pure parasitic loss. Worse, it rises exponentially with RPM, and is proportionally greater at lower power settings like cruise.