Smaller valve stems make room for air to pass past the valve. Its a trade off. More performance for slightly weaker valve.
What are you talking about?
The smaller valve stem makes for less valvetrain weight, meaning it can rev to higher RPM's with less inertia. On OHC engines valve stem thickness is not an issue, the stress is on top of the valve/hydraulic lifter.
If you are talking about airflow, the port smoothness and shape of the valve face (3 or 5 angle) are what affect airflow. Do not confuse people please.
What makes the E36 M42 superior to the E30 is definitely its potential to rev to higher RPM's safer. The intake tract and valvetrain are slightly improved, which is where the "6hp" come from. Internally they aren't much different, as far as bore/stroke are concerned. Also the fact that it has knock sensors allows you to advance ignition timing safely to the limit of your gas octane.