Not really following you on the cruise scenario. The only way you will get more airflow over the wings is if you increase your airspeed. You won't get anymore air over the wings with a headwind than if you do with a tailwind.
The climb performance is not improved. Headwind/tailwind has nothing to do with RoC, but does affect climb gradient (if you have a headwind you will cover less distance in the time it takes to climb to a given altitude).
Power + Attitude = Performance
In the windsheer case the aircraft has inertia. When acted upon by a gust of wind, the change in groundspeed is not immediate, the aircraft carries on at the same ground speed as it did before. Therefore the change in wind speed has a direct effect on the airspeed of the aircraft. Sudden tailwind, Reduction in airspeed - reduction in lift; sudden headwind, increase in airspeed - increase in lift.