AN ACR LGT TOUCHED DOWN DURING A GAR IN HVY RAIN AND WINDSHEAR.

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NEARING MIA; THE ATIS CHANGED 3 TIMES; MAINLY DUE TO SHIFTING WINDS; AND WE WERE FINALLY ASSIGNED THE ILS 27R. THE LAST ATIS RECEIVED PRIOR TO LNDG RPTED CEILING 3000 FT; VISIBILITY 1/2 HVY RAINSHOWERS; WIND 270/12 WITH A TSTM TO THE N OF THE FIELD. WE FOLLOWED A COMMUTER TURBOPROP AND AN ACR JET ON THE ILS 27R. ON TWR FREQ; THE TURBOPROP RPTED A 'SMOOTH RIDE ALL THE WAY DOWN;' AND ACR RPTED THAT 'THE RAIN STARTED 2 MI OUT; NOT A BAD RIDE.' AT ONE POINT THE TWR RPTED RVR ON 9L AS 4000 FT WITH NO RVR AVAILABLE FOR 27R. ON OUR APCH; WE SIGHTED THE LEAD- IN LIGHTS ABOUT 2 MI OUT IN LIGHT TO MODERATE RAIN. AT CAT 1 DECISION HT; THE LNDG ZONE AND CTRLINE LIGHT WERE VISIBLE WITH MODERATE RAIN OBSCURING VISIBILITY TO ABOUT 1/2 MI; MAYBE A BIT LESS. AT ABOUT 20 TO 30 FT RADIO ALT; THE ACFT STARTED A R DRIFT WITH A SIMULTANEOUS LOSS OF 20 KTS; FOLLOWED ALMOST IMMEDIATELY BY A TOTAL LOSS OF VISIBILITY IN HVY RAIN. I INITIATED A GAR; PUSHING THE THROTTLES FULL FORWARD WHILE THE CAPT GRABBED THE YOKE WITH ME TO PULL THE NOSE UP. WE FELT THE MAIN GEAR TOUCH DOWN RELATIVELY SMOOTHLY FOR 2 SECONDS. THE REMAINDER OF THE GAR WAS UNEVENTFUL; BUT WE ENDED UP ON ABOUT A 290 DEG HDG. THE MOST DANGEROUS POINT WAS WHEN WE HAD THE MAIN GEAR ON THE RWY WITH 0 FORWARD VISIBILITY. ALSO; WITH THE R DRIFT AND POST GAR HDG OF 290 DEGS; WE MUST HAVE BEEN PERILOUSLY CLOSE TO THE R EDGE OF THE RWY. I DON'T BELIEVE WE VIOLATED ANY FARS OR EXERCISED POOR JUDGEMENT. A SIT WITH NO RAIN ON A 2 MI FINAL AND HVY RAIN AT THE FILED MIGHT HAVE CONVINCED US TO DELAY OUR APCH; BUT THE FAVORABLE PIREP FROM AN ACFT 3 MI AHEAD SOUNDED GOOD. THE SHAFT OF HVY RAIN WAS OBSCURED TO US BY THE SURROUNDING MODERATE RAIN -- BOTH VISUALLY AND ON THE RADAR SCREEN. WE WERE SIMPLY UNLUCKY ENOUGH TO MEET THE HVY RAIN SHAFT; DOWNBURST; AND RAPID WIND SHIFT R IN OUR FLARE. I ALSO FEEL THAT THE CAPT WAS A BIT UNASSERTIVE PRIOR TO THE APCH BY NOT CLRLY DEFINING HIS 'PERSONAL' PARAMETERS AS TO WHEN TO INITIATE A GAR. BASED ON THE INFO AVAILABLE ; I THINK MOST AIRLINE PLTS WOULD HAVE FOLLOWED THE SAME COURSE WE DID. IF I HAD BEEN 1 OR 2 SECONDS QUICKER BY INITIATING THE GAR AT THE INSTANT I NOTICED THE R DRIFT; WE POSSIBLY COULD HAVE AVOIDED TOUCHING DOWN.