Cossack,
I think most pilots would disagree with the ATCO manual definition. The Boeing manual states "Severe windshear is that which produces airspeed changes greater than 15 knots." It doesn't have to be beyond the aircraft's performance capability to give you a fright.
It also states the following as indicating uncontrolled changes from the steady state condition ie windshear
Changes in excess of
15kts IAS
500fpm vertical speed
5 degrees pitch attitude
1 dot displacement from the glideslope
Unusual thrust lever position for a significant period of time.
I have the advantage that when I'm hand flying you would think you were in windshear the whole time. (doh)
"Is that a boy doggy Johnny?"