I did a quick flip through some flight tracks on 1Jan, and the fastest I could find was a B787 LAX-LHR. Peak ground speed was 735mph, with a filed speed of 564mph, giving a tail wind of about 171mph.
The total flight time was about 45mins shorter than average, because only a limited duration was spent inside the jet-stream. The track was longer in distance than the great-circle route, but still faster overall.
Obviously flights in the opposite direction use a more polar route to avoid the head winds.