ACR B727 FLC DSNDS FROM ASSIGNED ALT WHILE MANEUVERING FOR VISUAL APCH. PIC CONCERNED WITH COCKPIT WORKLOAD WHILE COMPLYING WITH ATC INSTRUCTION.

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ACFT #1 WAS ENRTE FROM SLC-LAS. AT 28 DME E OF LAS; WE WERE INSTRUCTED BY LAS APCH TO KEEP ACFT #2; AN MD80; AHEAD IN SIGHT AND MAINTAIN VISUAL SEPARATION; TURN TO 270 DEGS AND INTERCEPT THE RWY 25L LOC AT LAS. WE WERE TOLD TO TURN L TO 190 DEG HDG AT 6 DME FROM LAS VOR FOR A DOWNWIND VISUAL TO RWY 1R. IT WAS VERY HARD TO KEEP THE MD80 IN SIGHT; 2000 FT BELOW; AGAINST DESERT BACKGROUND. AT 15 DME LAS APCH ISSUED A DSCNT TO 8000 FT. PASSING 9000 FT; ATC ISSUED A TFC WARNING FOR DEPARTING TFC AND TOLD US TO SLOW TO 210 KTS. THE CAPT INFORMED THE FO WE WERE 1/2 MI FROM THE TURN POINT AND CONTINUED TO SEARCH FOR THE MD80. THE FO PULLED PWR TO SLOW DOWN AND STARTED A L TURN TO 190 DEGS. ATC THEN ADVISED OUR ASSIGNED ALT WAS 8000 FT. THE FO PULLED UP AND BOTH OF US NOTED 7650 FT ON OUR ALTIMETERS. WE RETURNED TO 8000 FT WITHIN 10 SECONDS AND WERE AT LEAST 2000 FT ABOVE OUR TFC. OPINION: ATC ISSUED INSTRUCTIONS TO KEEP VISUAL SPACING ON THE ACFT AHEAD; INTERCEPT A LOC COURSE ON ONE NAVAID; MAKE A TURN OFF A DME FROM ANOTHER NAVAID; DSND INTO CLBING TFC ON DEP AND SLOW FOR SPACING WHILE LEVELING IN A TURN. THE CREW WAS UNNECESSARILY OVERTASKED FLYING COMPLICATED INST PROCS WHILE ASKED TO MAINTAIN VISUAL SEPARATION FROM ACFT AHEAD ON APCH AND DEPS. ATC SHOULD EITHER ISSUE A VISUAL APCH CLRNC AND LET US MAINTAIN OUR OWN SEPARATION OR GIVE US AN INST APCH IF TFC WARRANTS.