A B757-200 PIC EXPERIENCES A POTENTIAL CONFLICT WITH DEP TFC AS HE TURNS TOO EARLY BECAUSE OF MISINTERP OF AMENDED DEP CLRNC AT TBPB; FO.

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LOCATION: BGI 090 DEG RADIAL; 1 DME. WHILE POS AND HOLD RWY 9; AMENDED CLRNC VECTOR HDG 345 DEGS GIVEN. AFTER TKOF; TURNED L TO 345 DEG HDG AT 400 FT PER CLRNC. CAPT KEPT FLAPS AT 15 DEGS TO EXPEDITE TURN. WHILE IN TURN; WE REALIZED THE LIGHT TWIN PISTON COMMANDER WOULD BE AHEAD OF US; WITH A SURPRISINGLY HIGH RATE OF CLB. THE CAPT REALIZED THE CONFLICT POTENTIAL AND CONTINUED HIS TURN FURTHER W TO A HDG OF 270 DEGS TO MAINTAIN SEPARATION BECAUSE A TURN TO THE E WOULD HAVE DIMINISHED THE EXISTING SEPARATION. IT WAS THEN WE BOTH REALIZED THE 345 DEG HDG AFTER TKOF WAS NOT INTENDED TO SUPERSEDE THE NOISE ABATEMENT PROC REQUIRING A CLB TO 2500 FT OR XING 7 DME BEFORE TURNING TO A N HDG. WE FAILED TO FOLLOW THE NOISE ABATEMENT PROC; IN ERROR; BELIEVING THE TWR CLRNC SUPERSEDED THE PUBLISHED PROC. MORE IMPORTANTLY; WE FAILED TO USE COMMON SENSE. WE SHOULD HAVE QUERIED THE TWR'S ISSUANCE OF A HDG AFTER TKOF THAT WOULD PUT US DIRECTLY BEHIND A SLOWER PISTON. CAPT AND I ERRANTLY ASSUMED WE COULD OUTCLB THE LIGHT TWIN. IN ADDITION TO OUR FAILURE TO USE COMMON SENSE; WE WERE MISLEAD BY THE UNITED STATES PROC WHEREBY AN AMENDED HDG AFTER TKOF MAY SOMETIMES SUPERSEDE THE PUBLISHED PROC. SUPPLEMENTAL INFO FROM ACN 496158: IT WAS THEN THAT WE BOTH REALIZED THE DEP HDG WAS NOT INTENDED TO SUPERSEDE THE NOISE ABATEMENT RESTR ON TURNS BELOW 2500 FT OR 7 DME. ATTAINING EITHER ONE WAS THE APPROPRIATE TIME TO TURN TO 345 DEGS. AT THIS POINT; IT WAS TOO LATE TO TURN BACK. I CONTINUED TO TURN TO 270 DEGS TO STAY AWAY FROM THE OTHER ACFT WHILE WE MAINTAINED VISUAL CONTACT. WE CROSSED HIS ALT SHOWING JUST OVER 2 MI SEPARATION ON TCASII -- AGAIN; IN VISUAL CONTACT.