People have installed it with the blue spring (18psi) and noticed the difference. Boost comes on strong and is much smoother as its more controlable and the boost controller isnt going nuts trying to keep it at the required pressure. The fluttering that you are probably hearing will be gone as the actuator and N75 valve arent going mental trying to hold more boost.
Installing is a prick. Oil and coolant needs to be drained (well most of the coolant) turbo off. Full days work. Its expensive for a $200 part but for some its worth it. Most who have done it have had the skills to do it themselves which always helps.
What state are you in. Add it to your profile to help the brothers out.