SFO LCL CTLR EXPERIENCES CONFLICT AT 1200 FT WHEN IFR RWY 28R DEP CONFLICTED WITH VFR TFC WATCH FLT; VISUAL SEPARATION IN USE.

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ACFT X F2TH DEPARTING ON SHORELINE DEP RWY 28R VERSUS ACFT Y C172 ON TFC WATCH. I ISSUED ACFT Y AT OR BELOW 1500 FT AND REMAIN E OF HWY 101 FROM NW OF THE ARPT; TRANSITIONING SEBOUND. WHEN ACFT X BEGAN CLBOUT; I INSTRUCTED ACFT Y TO MAINTAIN VISUAL SEPARATION. AT THAT POINT; ACFT Y WAS DSNDING THROUGH 1200 FT TO A NEW ALT RESTR AT OR BELOW 1000 FT; AND ACFT X WAS CLBING THROUGH 1200 FT. THEY WERE 2 MI APART. THE F2TH; AFTER ADVISED C172 WAS MAINTAINING VISUAL SEPARATION; COMPLAINED THE C172 WAS IN 'ABOUT THE WORST POSSIBLE PLACE HE COULD BE.' 2 THINGS OCCURRED: 1) THE C172 DID NOT DO AS INSTRUCTED -- E OF HWY 101. THEY WERE MORE OR LESS OVER THE HWY. I AMENDED INSTRUCTIONS TO 1 MI E OF HWY 101; AT OR BELOW 1000 FT WHEN I SAW IT WAS GOING TO BE CLOSER THAN I LIKED. 2) THE F2TH WAS CLBING IN A LESS THAN EXPECTED PERFORMANCE; AND WHEN ADVISED 'TFC 2 MI N DSNDING THROUGH YOUR ALT HAS YOU IN SIGHT MAINTAINING VISUAL SEPARATION;' DID NOT SOUND SATISFIED. I HAVE SEEN MANY TIMES CLOSER TRANSITION VERSUS 28 DEP/GAR SITUATIONS. I FELT THE OP WAS VERY SAFE; WITH MY AMENDED INSTRUCTIONS. I ADVISED ACFT X THAT WE (SFO TWR) TRIED TO ELIMINATE THESE TRANSITIONS; BUT PLT GROUP WENT ABOVE US AND WE (CTLRS) WERE TOLD WE COULD NOT BLANKET IGNORE TRANSITIONS. IF THERE IS GOING TO BE AN ACFT COLLISION AT SFO IT WILL BE A TRANSITION AND A DEP/GAR.