Changing the relief spring will change pressure for cruising/WOT only...for idle you need to set the pump gear clearance tighter or try a booster plate...I use an adjustable relief spring setup from T/A Perf...just loosen the lock nut and turn screw with allen wrench...some may think a seup like that might leak but I've been using it on both my cars for years and it hasn't leaked a single drop
We are using one of these on the Turbo Z for external oil pump pressure control. We goofed around with springs for a long time and finally gave up. You can adjust this type of valve any time you want. It is actually for Hydralic pressure control for machinery, you won't find it sold by any performance companies most likely.