A wide LSA does not affect the idle quality, the idle quality improves because of the reduction in overlap. The engine does not know or care what the LSA is, only the opening and closing events of the camshaft. You can adjust the overlap using the Intake duration, the Exhaust duration, the LSA, or all of the above in combination. Increasing LSA by 2 degrees reduces the overlap by 4 degrees total, but also moves every other timing event for the cam. The optimum overlap for any particular combo will "start" the drawdown of clean air on the intake stroke and "clean" the chamber of all residual exhaust gases on the exhuast stroke. Too much overlap leads to reversion, lazy throttle response, low dynamic compression, poor idle. Too little overlap sacrifices mid range torque and increases the sensitivity to detonation.