IFR PC12 DEP FROM BED IS FORCED TO MAKE EARLY TURN BECAUSE OF PRECEDING TFC AND ATC IMMEDIATE TKOF CLRNC.

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WE WERE OPERATING A PILATUS PC12 IN BEDFORD; MA (BED). WE CONTACTED THE TWR AND TOLD THEM WE WERE HOLDING SHORT OF RWY 23 ON TXWY H AND WE WERE READY FOR TKOF. THERE WAS CURRENTLY A MOONEY IN POS AND HOLDING ON THE RWY. WE WERE INFORMED TO 'HOLD SHORT.' APPROX 1 MIN LATER THE MOONEY WAS CLRED FOR TKOF. A FEW SECONDS LATER WE WERE CLRED FOR 'IMMEDIATE' TKOF. WE OBSERVED ANOTHER PILATUS ON ABOUT A 1 MI FINAL. WE PULLED ONTO THE RWY TO BEGIN OUR GND ROLL; BUT HAD TO STOP DUE TO THE SLOW MOVING MOONEY IN FRONT OF US. WE DIDN'T WANT TO SIT ON THE RWY AS OUR CLRNC WAS OR AN 'IMMEDIATE' TKOF; BUT IT WAS NOT POSSIBLE TO GO. WE PAUSED FOR SEVERAL SECONDS TO GIVE THE MOONEY TIME AND STARTED OUR TKOF ROLL. ONCE AIRBORNE WE HAD TO START OUR TURN TO OUR ASSIGNED HEADING AT ABOUT 300 FT DUE TO THE PROX OF THE MOONEY. CLRLY I SHOULD HAVE DECLINED THE TKOF CLRNC DUE TO THE PILATUS ON FINAL. IT WOULD NOT HAVE BEEN A PROB HAD THE ACFT ON THE RWY BEEN ONE OF SIMILAR PERFORMANCE. IF WE HAD BEEN OPERATING AT AN UNCTLED FIELD I WOULD HAVE NEVER ATTEMPTED THIS TKOF; BUT BECAUSE THE CTLR HAD GIVEN ME THE 'CLRNC' I ELECTED TO CONTINUE. IT SEEMS TO ME THAT CTLRS SHOULD NEVER GIVE A CLRNC FOR AN IMMEDIATE TKOF WHEN ANOTHER ACFT WITH MUCH SLOWER PERFORMANCE HAS JUST DEPARTED. PLTS RECEIVING SUCH A CLRNC SHOULD CONSIDER THE PREVIOUSLY DEPARTING ACFT FOR POSSIBLE CONFLICTS DUE TO PERFORMANCE. THE WISDOM OF ISSUING AN 'IMMEDIATE' CLRNC OF THIS TYPE SHOULD BE QUESTIONED BY CTLRS.