B757 HAS TCASII RA AND TFC CONFLICT AT ATL WHEN ANOTHER ACFT IS VECTORED ACROSS FLT PATH.

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WE WERE ON L DOWNWIND FOR RWY 8L AT ATL AT 11000 FT; 210 KTS. ATC CLRED US TO 7000 FT; SLOW TO 170 KTS. CTLR THEN TOLD US TO MAINTAIN 210 KTS AND FREQ CHANGE. FINAL CTLR CLRED US TO 5000 FT WITH 5 DEG R TURN (275 DEG HDG). WE WERE ABOUT 2 MI BEHIND ANOTHER B757 WHO WAS AT OUR 1 O'CLOCK POS. I EXPEDITED THE DSCNT TO 5000 FT EXPECTING TO BE TURNED IN FIRST. I WAS UNCOMFORTABLE WITH THE SPACING AND BEGAN TO SLOW TO 180 KTS. ATC THEN TURNED THE OTHER ACFT TO 180 DEGS ACROSS OUR PATH. HE WAS STILL DSNDING TO 5000 FT BUT OUR TCASII COMMANDED 'CLB.' I TURNED ABOUT 15 DEGS R TO FLY BEHIND HIM RATHER THAN PULL UP INTO HIM STRAIGHT AHEAD. ONCE CLR; I RETURNED TO MY ASSIGNED HDG. I BELIEVE THE FINAL CTLR ASSUMED WE WERE AT 170 KTS NOT 210 KTS AND WAS UNAWARE OF THE DECREASED SEPARATION. HAD WE BEEN DIRECTLY BEHIND THE LEAD ACFT; THE PROB WOULD NOT HAVE BEEN AS GREAT. SUPPLEMENTAL INFO FROM ACN 455329: THE CTLR SHOULD HAVE NOTED OUR OVERTAKE; THEN EITHER ISSUE US A SLOWER SPD; OR COULD HAVE TURNED US TO BASE LEG FIRST. ADDITIONALLY; A MORE TIMELY TA WOULD HAVE HELPED. IT REALLY DIDN'T DO MUCH TO HELP WITH THE TFC DIRECTLY IN FRONT OF US AND AN RA GOING OFF AT THE SAME TIME.