TCASII RA COMMANDS EXCESSIVE DSCNT WHICH APPEARED TO RPTR TO BE ERRONEOUS.

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WE WERE CLBING OUT OF MEM ON THE MEM 275 DEG RADIAL AT APPROX 30 MI FROM MEM WHEN WE RECEIVED A TCASII TA. AFTER SELECTING A CLOSER RANGE WE SAW AN AMBER DIAMOND SHOWING TFC AT 11-11:30 O'CLOCK; 2 MI AND OUR ALT; 11000 FT. IMMEDIATELY THE CAPT REQUESTED A VECTOR FROM ATC FOR TFC AVOIDANCE; AND COPLT PREPARED TO TURN ACFT TO AVOID TFC. WE THEN GOT A TCASII RA COMMANDING A 1500 FPM DSCNT TO GET INTO THE 'GREEN' AREA. THE FO (ME) IMMEDIATELY BEGAN A 1500 FPM DSCNT TO COMPLY WITH THE ADVISORY WHEN WE GOT THE MESSAGE 'INCREASE DSCNT.' I PUSHED THE NOSE OVER TO COMPLY AND GET BACK INTO THE GREEN; BUT AT 7000 FPM DSCNT; THERE WAS ONLY RED (IE; TCASII WAS COMMANDING HIGHER DSCNT RATE). AT THIS POINT I SPOTTED THE OTHER ACFT AT 10 O'CLOCK AND 1/2 MI STILL AT OUR ALT. I CONTINUED AN AGGRESSIVE DSCNT AND MADE A SLIGHT R TURN TO AVOID THE CONFLICTING ACFT. THE OTHER ACFT APPEARED TO BE A CESSNA 172 OR 150. HE PASSED APPROX 1/4 MI TO THE L AND 200 FT ABOVE US. I BELIEVE THE TCASII ADVISORY WAS WRONG. IF WE HAD INCREASED OUR CLB RATE; I BELIEVE WE COULD HAVE AVOIDED THE TFC WITHOUT INCIDENT. AS IT WAS; IT TOOK AN AGGRESSIVE MANEUVER AND ACFT OVERSPD (370-375 KTS) TO AVOID THE OTHER ACFT. EARLIER ATC WARNING AND VECTORING WOULD ALSO HAVE HELPED. OTHER NOTES: WE RECOVERED FROM DSCNT AT APPROX 9000 FT MSL. MAX SPD IN DSCNT 370-375 KTS. MAX DSCNT RATE 7000-13000 FPM. NO 'G' LIMITS EXCEEDED. NO PAX INJURED. ATC NOTIFIED DURING AND AFTER RA.