PLT OF P51 FLT WITH ZAU AT 11500 FT NEAR ORD CAUSES TCASII RA WITH ACR AT 11000 FT; ATC ISSUED TFC.

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I WAS ONE OF THE PLTS IN A FLT OF 2 P51'S; VFR AT 11500 FT N OF CHICAGO ORD; TALKING WITH ZAU. WE WERE ON THE SAME FREQ WITH ACFT Y WHO WAS LEVEL AT 11000 FT. HE WAS ISSUED TFC (12 O'CLOCK POS AT 11500 FT) AND ACKNOWLEDGED. WE SAW HIM WHEN HE WAS SEVERAL MI AWAY; THE CTLR ISSUED TFC TO ACFT Y AGAIN AT 12 O'CLOCK POS; AND 1 MI; HE RESPONDED: THAT THEY HAD AN RA; HAD DSNDED; HAD US IN SIGHT AND WERE CLBING BACK TO 11000 FT. I BELIEVE THAT ALL 3 PARTIES INVOLVED IN THIS SIT ACTED IN ACCORDANCE WITH THE FAR'S. WE WERE LEGAL VFR AT 11500 FT; ACFT Y WAS CORRECT TO FOLLOW THE RA; THE CTLR (I ASSUME) WAS CORRECT IN ISSUING THE TFC AND NOT GIVING ONE OF US A TURN. HOWEVER; AS A B767 CAPT; FAMILIAR WITH TCASII; I'M CERTAIN THAT ACFT Y WOULD HAVE PREFERRED A SLIGHT TURN RATHER THAN RESPONDING TO A TCASII ALERT! WE WOULD HAVE BEEN MORE THAN HAPPY TO TAKE A TURN FOR TFC. BETTER JUDGEMENT ON THE PART OF THE CTLR WOULD HAVE BEEN TO 'DECONFLICT' THE TFC; OR AT LEAST OFFER A TURN TO ONE OF US! WE PREFER NOT TO CROSS THE LAKE IN A SINGLE ENG ACFT OF THIS TYPE AND APPRECIATE THE ABILITY TO TRANSITION THE CHICAGO AIRSPACE. THE AIRSPACE COULD BE SHARED WITH A HIGHER MEASURE OF SAFETY; IF THE CTLR WERE ENCOURAGED TO OFFER A TURN. I UNDERSTAND THAT TFC SEPARATION CANNOT BE DICTATED WITH VFR TFC. BUT AGAIN; COULD THEY BE ENCOURAGED TO OFFER THE TURN TO AVOID A DEV BY AN AIRLINE ACFT WHEN THE VFR IS WILLING TO COOPERATE FOR INCREASED SAFETY?