I'm not sure of all the rules about modifications, but I'm sure that one is allowed. That modification, (changing the recoil spring), is to improve function and reliability, not to gain an "unfair" advantage.
Give me a bit, and I'll verify it.
Here is a link I found on another thread. You can find what you're looking there. http://www.idpa.com/
Blessed are they who, in the face of death, think only about
the front sight.