VNAV takeoff (armed on the ground, active at) is the standard boeing SOP since FMC 8.1. And yes, it does command 230kias until the LEDs are retracted. From what i heard is the 20kts buffer simply a safety buffer to the 250kias limit for sloppy flying. Especially if the autopilot is already active and the plane encounters a windshear the ability of the autopilot to recover would not be sufficient if VNAV commands 250kias.