Lots of causes for sponginess, some of which will only be peripherally addressed by upgrading the system.
Air in the lines is the #1 reason.
Air in the proportioning valve is the #2 reason.
Air in the lines is the #3 reason.
Old and tired flex lines are the #4 reason.
The floor-board flexing when you push the brake pedal is probably #5.
There are fixes for all of these; bleeding the air out is great for #1 and #3 (and #6-20
). Aftermarket proportioning valves don't tend to trap air as readily as the stock one, so they may not need to be directly bled. Floor-pan reinforcements can help the flex, as can bracing the master cylinder.
Upgrades to the system can change the brake balance, and will certainly change the heat capacity for the system.
--DD