B737-300 FLC USES THE WRONG GROSS WT FOR THE ACFT PERFORMANCE CALCULATIONS; RESULTING IN A HARD LNDG AT HOU; TX.

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THE LOADSHEET WAS GIVEN TO US FOR AN ON TIME PUSHBACK AND THE FO LOADED THE NUMBERS IN THE PERFORMANCE COMPUTER AND CDU PER NORMAL OPS. NO DISCREPANCIES WERE NOTED AT THIS TIME; HOWEVER; I THOUGHT THAT THE V-SPDS SEEMED LOWER THAN WHAT THEY SHOULD HAVE BEEN (CLUE #1). THE PAX COUNT ON THE LOADSHEET AND THE FLT ATTENDANT PAX COUNT MATCHED. PUSHBACK AND ENG START WERE NORMAL; FOLLOWED BY A SHORT TAXI TO RWY 1 FOR TKOF. TKOF WITH FLAPS 5 DEGS AND REDUCED THRUST OF 85% PER THE PERFORMANCE COMPUTER WERE NORMAL. WE INITIALLY SET THE THROTTLES AT 90% AND LEFT THEM THERE. THIS ACTUALLY HELPED OUR TKOF PERFORMANCE BUT ALSO PROBABLY HELPED HIDE THE WT DISCREPANCY. SINCE IT WAS THE FO'S LEG I DID NOT NOTICE ANYTHING UNUSUAL ABOUT THE TKOF OR ACFT PERFORMANCE. THE FO LATER INFORMED ME THAT THE ACFT SEEMED TO FLY LIKE A HEAVIER ACFT THAN WE HAD CALCULATED; BUT THAT HE THOUGHT THE TRIM SETTING WAS A LITTLE OFF (CLUE #2). CLB AND CRUISE SEEMED NORMAL; HOWEVER; THE DSCNT PROFILE WE FLEW WAS A LITTLE BEHIND THE REQUIRED DSCNT PROFILE TO MAKE XING ALTS (CLUE #3). HOU APCH TURNED US INSIDE DOWNTOWN HOU; FROM A L DOWNWIND FOR RWY 12R; AND SLIGHTLY HIGH. WE WERE HAVING TROUBLE SLOWING THE ACFT; GETTING IT CONFIGURED AND DSNDED ON THE APCH WITH FLAPS 30 DEGS AS BRIEFED (CLUE #4). WE ELECTED TO USE FLAPS 40 DEGS; BUT STILL COULD NOT MEET THE STABILIZED APCH CRITERIA; SO WE ASKED FOR RWY 4. IT TOOK SEVERAL ATTEMPTS TO REQUEST RWY 4 FROM APCH CTL BEFORE WE WERE ANSWERED. THEY SENT US TO HOU TWR AND THEY WERE UNABLE TO GIVE US RWY 4 SO WE ELECTED TO GO AROUND. THIS PROBABLY HELPED OUR PERFORMANCE SINCE THE FUEL USED ON THE GAR LOWERED OUR GROSS WT FOR THE SUBSEQUENT APCH AND LNDG ON RWY 12R. ON THE SECOND APCH TO RWY 12R WE BOTH NOTICED THAT THE ACFT WAS UNUSUALLY NOSE HIGH FOR FLAPS 30 DEGS AND THAT MORE PWR THAN USUAL WAS REQUIRED TO MAINTAIN OUR CALCULATED APCH SPD (CLUE #5). I RECHKED THE FLAP SETTING AT LEAST TWICE TO MAKE SURE THE FLAPS WERE SET CORRECTLY. WE ACTUALLY FLEW 10 KTS FASTER THAN OUR CALCULATED APCH SPD IN ORDER TO HAVE BETTER CTL OVER OUR PITCH ATTITUDE. ONCE DURING THE APCH I NOTICED THE STALL INDICATOR APPEAR AT THE TOP OF THE HUD VIEWFINDER AND THEN GO AWAY (CLUE #6). AT THIS POINT WE KNEW THAT SOMETHING WAS WRONG; BUT EVEN WITH ALL THE CLUES WE DID NOT KNOW WHAT IT WAS. THE APCH ENDED WITH A HARD LNDG. AT THIS POINT; IT FINALLY DAWNED ON ME THAT PERHAPS OUR WTS WERE WRONG ON THE LOADSHEET THEREFORE WRONG IN THE PERFORMANCE COMPUTER AND CDU. WHEN WE CHKED THE LOADSHEET WE REALIZED THAT THE AGENT IN MSY HAD NOT ADDED THE PAX AND CARGO WT TO THE OEW AND HAD USED THE OEW AS THE ZFW. THIS RESULTED IN A 22000 LB ERROR IN OUR PERFORMANCE CALCULATIONS.