B757 FLT CREW ENCOUNTERS WINDSHEAR ON FINAL APCH TO ZZZ AND EXECUTES A GAR.

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DURING APCH TO ZZZ; ATC ADVISED OF MODERATE TO SEVERE WX RETURNS ON OR NEAR OUR APCH PATH. WE WERE IMC; AND EXPERIENCING LIGHT PRECIP AND LIGHT CHOP. THERE WAS NO LIGHTNING VISIBLE DURING THE ENTIRE EVENT. RADAR RETURNS DID NOT INDICATE SEVERE WX ON OUR FLT PATH; HOWEVER; THE WX RETURNS WERE JUST W OF OUR FINAL APCH PATH. EVEN THOUGH ATC REPEATEDLY ADVISED US OF THE SEVERE WX VISIBLE ON OR NEAR OUR APCH; WE CONTINUED DUE TO OUR RELATIVELY SMOOTH AND UNEVENTFUL APCH. WE WERE THE FIRST ACFT ON THE FINAL (NO PIREPS FROM ANY ACFT AHEAD OF US). SEVERAL TIMES DURING THE APCH; THE CAPT AND I DISCUSSED OUR WILLINGNESS TO GO AROUND IF ANYTHING MADE US UNCOMFORTABLE. AT APPROX 1000 FT AGL WE WERE NOT STABLE (AIRSPD 40 KTS TOO HIGH) AND WE BOTH AGREED ON A GAR. DURING THE GAR WE EXPERIENCED SIGNIFICANT WINDSHEAR (AIRSPD GAIN OF APPROX 40 KTS); AND MOST LIKELY OVERSPED THE FLAPS BY 10-15 KTS. (OVERSPEED ENTERED IN LOGBOOK). THE GAR WAS NOT PRETTY DUE TO WINDSHEAR; AND LOW ALT CAPTURE; BUT ACFT WAS UNDER POSITIVE CTL. WE WERE SLOW TO RETRACT FLAPS DUE TO HIGH WORKLOAD ASSOCIATED WITH WINDSHEAR AND CLBING-TURNING; GAR MANEUVER. WAS FATIGUE A FACTOR? PROBABLY; DUE TO OUR CIRCADIAN RHYTHM SHIFTS. WE WERE LAUNCHED AS A STANDBY CREW; AND THEN GIVEN A MINIMUM LAYOVER (9 HRS). THIS WAS FOLLOWED BY FLT DURING A TIME WHEN I WOULD NORMALLY BE SLEEPING. ALL OF THIS CAN HAVE A SUBTLE EFFECT ON YOUR BETTER JUDGEMENT. BOTTOM LINE: WE SHOULD HAVE BROKEN OFF THE APCH EARLIER. THERE WERE PLENTY OF HINTS THAT CONTINUING WASN'T A GREAT IDEA. ATC; OUR RADAR AND OUR GUT WERE ALL IN AGREEMENT; BUT FOR SOME REASON WE CONTINUED. WE HAD PLENTY OF GAS; AND WE WERE NOT IN A HURRY. NEXT TIME; HOLDING WILL BE MY PLAN OF ACTION.