MD80 CREW ENCOUNTERED WAKE TURB BEHIND A B757-200 WHICH ALMOST EXCEEDED THE CTL CAPABILITY OF THE ACFT.

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WAKE TURB ENCOUNTER. LOCATION: PRIOR TO GS ON RWY 22R ILS APCH. WE WERE IMC ON ILS FINAL TO RWY 22R AT ORD AT 5000 FT AND CLRED FOR THE APCH. WE HAD BEEN SLOWED DOWN TO 160 KTS AND ADVISED TO USE CAUTION FOR WAKE TURB AS WE WERE FOLLOWING A B757. SPACING WAS APPROX 4.5 MI ON OUR TCASII SCOPE. THE B757 AHEAD OF US WAS FLYING 500 FT HIGH ACCORDING TO OUR TCASII ALT INDICATION. THE FO AND I COMMENTED THAT WE WERE GOING TO HIT HIS WAKE; AND THAT WE WOULD BE READY FOR IT. I BELIEVE THAT THE B757 WAS FOLLOWING ANOTHER B757 AND WAS PROBABLY FLYING HIGH TO AVOID ITS WAKE. WHEN WE HIT THE WAKE; IT EXCEEDED THE ABILITY OF OUR AUTOPLT TO CTL THE ACFT. THE AUTOPLT DISCONNECTED AND I HAND FLEW AS THE ACFT ROLLED APPROX 25 DEGS L AND R -- CERTAINLY ENOUGH TO CONCERN OUR PAX. I INFORMED APCH CTL THAT WE WERE GOING TO CLB 200 FT AND TOLD HIM WHY. HE SAID OK; BUT COMMENTED THAT THE SPACING WAS GOOD. I RESPONDED THAT I WAS NOT COMPLAINING ABOUT THE SPACING BUT THAT WE WERE BEING 'BEAT UP' BY THE WAKE. I WAS PREPARED TO FLY 1/2 DOT HIGH ON THE GS IF NECESSARY TO AVOID HIS WAKE; BUT ONCE ON THE GS; THE WAKE WAS NOT A PROB. UPON LNDG; I CALLED THE APCH FACILITY ON THE HOTLINE IN ORD TO EXPLAIN WHAT HAD HAPPENED. THE PERSON I SPOKE TO SAID HE WOULD PASS IT TO THE CTLR. THE PURPOSE OF THIS RPT IS NOT TO CRITICIZE THE CTLR; BUT TO POINT OUT THE AFFECT OF A B757 ON AN MD80 IN THAT SIT.