CAPT ON B737-900 RPTS RWY LIMITED TKOF PERFORMANCE NEAR MAX GROSS WT WAS SERIOUSLY DEFICIENT. SUSPECTS ACFT WT WAS GREATER THAN SHOWN ON THE WT MANIFEST.

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FO ROTATED TO ESTABLISH TKOF ATTITUDE AT VR; ACFT CONTINUED TO ROLL FOR SOME TIME PRIOR TO BECOMING AIRBORNE. AFTER BECOMING AIRBORNE; THE FO PERFORMED FURTHER ROTATION TO ESTABLISH INITIAL CLB ATTITUDE. THE ACFT SEEMED SLUGGISH IN GETTING A RATE OF CLB ESTABLISHED. ONCE SAFELY AIRBORNE AND OUT OF THE ARPT AREA; WE BOTH REVIEWED WHAT HAD TRANSPIRED. IT SEEMED THERE WAS PRECIOUS LITTLE RWY REMAINING WHEN THE ACFT BECAME AIRBORNE; AND I FELT THERE WAS A VERY REAL CHANCE THAT WE MAY IMPACT THE ELEVATED APCH LIGHTS FOR RWY 14 DURING THE INITIAL CLB SEGMENT. I HATE TO THINK WHAT WOULD HAVE HAPPENED HAD WE LOST AN ENG AT OR NEAR V1. I SHOULD HAVE REQUESTED A LOAD AUDIT ON OUR ARR IN SEA; I CAN ONLY SURMISE THAT WE WERE HEAVIER THAN WE THOUGHT. IN THE FUTURE WHEN DEPARTING IN A HVY ACFT; I INTEND TO REQUEST THE LONGEST AVAILABLE RWY REGARDLESS OF WHAT THE NUMBERS SAY WE CAN DO. IF WE HAD LOST AN ENG JUST PRIOR TO V1; I SUSPECT WE WOULD HAVE OVERRUN THE RWY IN THE STOP ATTEMPT; AND HAD WE LOST AN ENG AT OR RIGHT AFTER V1 WE MAY NOT HAVE GOTTEN AIRBORNE AND IF WE HAD I SUSPECT WE WOULD HAVE IMPACTED THE RAISED APCH LIGHTS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT WAS LESS THAN ENTHUSIASTIC ABOUT THE GENERAL PERFORMANCE OF THIS; THE LONGEST OF ALL B737'S. THE FUSELAGE HAS BEEN STRETCHED OVER 40% FROM THE EARLIEST VERSIONS AND THE PAX LOAD ALMOST DOUBLED. HE INDICATED THIS EXPERIENCE WAS EXTREME ENOUGH THAT IT WOULD IMPACT EVERY DECISION HE MAKES ABOUT WT LIMITED TKOFS IN THE FUTURE. IN PARTICULAR; THE USE OF INTXN TKOFS ON RWYS VERSUS USING THE ENTIRE LENGTH REGARDLESS OF THE IMPACT ON ARPT OPS OR DEP DELAYS. HE STRESSED HIS BELIEF THAT; HAD AN ENG FAILED AT OR NEAR V1; THEY WOULD HAVE BEEN UNABLE TO STOP ON THE REMAINING RWY OR BECOME AIRBORNE PRIOR TO ITS END. HE ALSO EMPHASIZED THE FACT THE AMBIENT TEMP WAS SEVERAL DEGS COOLER THAN THAT ON WHICH THE RWY PERFORMANCE NUMBERS WERE BASED AND THERE WAS A HEADWIND COMPONENT; BOTH OF WHICH SHOULD HAVE IMPROVED THE ACTUAL PERFORMANCE IF THE WTS WERE ACCURATE. DESPITE THOSE PERFORMANCE ENHANCING FACTORS THE ACTUAL TKOF WAS DISTURBING ENOUGH TO GENERATE THE RPT.