CL601 FLC CONCERNED ABOUT CLB CLRNC.

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WE HAD JUST BEEN RETURNED TO ON COURSE DIRECT YXU (LONDON; ONTARIO) AFTER BEING ON AN OFF-COURSE VECTOR FOR TFC DUE TO OUR SLOW CLB ON OUR WAY TO FL270. CTLR PREVIOUSLY ASKED IF WE COULD INCREASE OUR CLB RATE AND WE ANSWERED THAT WE COULDN'T; AND THAT WE WERE ALSO AT OUR MINIMUM CLB SPD (250 KIAS). IN THE DIRECT YXU CLRNC; ATC ALSO STATED 'NO DELAY TO FL270.' THIS WAS AS WE PASSED FL260. AT FL265; OUR CLB RATE DETERIORATED FURTHER; TO LESS THAN 500 FPM; AND I NOTIFIED THE CTLR. CTLR SIMPLY SAID WE NEEDED TO BE LEVEL AT FL270 -- DID NOT OFFER A VECTOR OR POINT OUT TFC. THEN WE GOT A TA; WITH HEAD-ON TFC APCHING AT FL260. I THINK WE WERE IMC. WE INCREASED OUR CLB RATE; EVEN THOUGH GOING BELOW OUR COMPANY-APPROVED MINIMUM CLB SPD. WE REACHED FL270 WITH THE OPPOSITE DIRECTION TFC JUST OUTSIDE THE 5 MI RING ON TCASII; PERHAPS AT 5 1/2 - 6 MI; BUT DEFINITELY OUTSIDE THE 5 MI RING. FOR SOME REASON; PERHAPS THE ADJUSTMENT TO CLB RATE SO CLOSE TO OUR CAPTURE ALT; AUTOPLT DID NOT CAPTURE AT FL270. DUE TO DISTR OF CONFLICTING TFC; WE CONTINUED ABOVE FL270; REACHING MAX FL272 AS WE ADJUSTED PITCH TO DSND BACK TO FL270; WHICH THEN CAPTURED. NO APPARENT TFC CONFLICTS DUE TO THIS OVERSHOOTS; NO TFC ABOVE US ON TCASII. ALTHOUGH STANDARD SEPARATION WAS NOT LOST; IT WAS CLOSE ENOUGH TO WARRANT A RPT. I CANNOT UNDERSTAND WHY THE CTLR; KNOWING WE WERE A SLOW CLBER; TURNED US BACK IN THE DIRECTION OF HEAD-ON TFC; INSTEAD OF KEEPING US AWAY UNTIL WE HAD REACHED FL270. I CANNOT UNDERSTAND WHY THE CTLR; AFTER BEING NOTIFIED OF FURTHER DEGRADING CLB; DID NOT OFFER A TURN; AND/OR GET OTHER TFC TO TURN AS WELL. WE DID THE BEST WE COULD; AND LET THE CTLR KNOW THERE WAS NO BETTER WE COULD GIVEN HIM. STILL; WE WERE PUT IN THIS SIT. WERE WE TO QUESTION THE VECTOR BACK ON COURSE? WE ARE NOT THE ONES IN THE POS TO KNOW THE OTHER TFC FACTORS; SO I THINK NOT. I DON'T NOW WHAT ELSE W COULD HAVE DONE.