BA41 PIC CONCERNED WITH ATC HANDLING WHILE ON APCH TO STL ARPT. ALT AND SPD ASSIGNMENT APPEARS TO CONFLICT WITH FARS; SPECIFIC TO TURBOPROP OP.

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WE WERE ASSIGNED 4000 FT WHILE ON THE LOC FOR RWY 24 OUTSIDE CLASS B AIRSPACE. AIRSPD WAS REQUESTED BY ATC AT APPROX 22 NM. WE RPTED OUR SPD AS 230 KTS WHILE DECELERATING. WE CONTINUED AT 4000 FT BELOW CLASS B 'STEP' (15-20 NM; 5000 FT). MY CONCERN FOR RPT IS THIS IS A VERY COMMON PRACTICE AT STL OF VECTORING AND ASSIGNING ALTS BELOW CLASS B LATERAL LIMITS. THE BAE 4100 IS A LARGE TURBINE PWRED ACFT; AND AS THIS IS AN APPROVED PROC UNDER THE FARS FOR ATL; THIS ALSO PLACES (I FEEL) AN UNNECESSARY BURDEN ON THE AIR CREW OF OBSERVING SPD RESTRS IN AND UNDER CLASS B AIRSPACE. IN AN ENVIRONMENT WHERE ATC IS ASKING US TO GIVE MAX FORWARD SPD THE MAJORITY OF THE TIME; ASSIGNING AN ALT WITHIN CLASS B WOULD PERMIT AN ADDITIONAL 50 KTS AND MOST IMPORTANTLY; REDUCE THE RISK OF FLC VIOLATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED THAT ATC FREQUENTLY PLACED TURBOJETS BENEATH CLASS B; ASKING FOR 'MAX FORWARD SPD;' WITHOUT THE UNDERSTANDING OF THE FAR REQUIREMENT OF REDUCED SPD; 200-210 KTS; WHEN OPERATING BENEATH THE FLOORS OF CLASS B. THE RPTR ALSO HAD CONCERNS OF BEING SET UP BY APCH CTL TO ONE RWY; THEN CHANGED ON INITIAL CONTACT BY THE TWR TO THE OTHER PARALLEL RWY OR TO RWY 13/31 -- A CONVERTED TXWY TO A RWY. THE RPTR ADVISED THAT FREQUENTLY THERE WERE CONCERNS OF WAKE TURB THAT HE PERCEIVED THAT ATC WAS NOT TAKING INTO ACCOUNT.