The managed speed looks after the takeoff (V2+10 knots) during SRS until acceleration altitude (CLB). If left in managed, the aircraft would then accelerate (through pitch down) to the SPD LIM (250<10,000 ft). Pre-selecting green dot speed allows the autopilot to automatically open the window for you at acceleration altitude and climb out at green dot, which is your best lift/drag ratio speed, and your minimum maneuver speed. This also gives you the best climb gradient, which should be used until minimum safe altitude (MSA). After MSA, pushing for managed allows for acceleration to 250 knots, until 10,000 feet and then it automatically bugs for ECON SPD. The pre-selection on the PERF page only saves you the trouble of manually setting green dot for best climb angle. Does that help you?