B737-700 FLT CREW EXCEEDS SPD RESTR BELOW 10000 FT AND CLASS B AIRSPACE.

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FLYING TO SNA VIA COAST 5 DEP. 37 MINS LATE DUE TO UNSCHEDULED ACFT SWAP. CLRED TO DSND TO 5000 FT APCHING SXC. THE FO ASKED IF HE COULD KEEP HIS CRUISE DSCNT SPD UP BELOW 10000 FT SINCE WE WERE MORE THAN 12 MI OFFSHORE. I SAID 'GOOD IDEA.' IT WAS A BAD IDEA. AS WE WERE GIVEN A VECTOR TOWARD SNA; WE WERE GIVEN A FREQ CHANGE AND RECEIVED POP-UP TCAS TFC AT THE SAME TIME. I BUMBLED THE FREQ CHANGE WHILE LOOKING FOR VFR TFC AND BY THE TIME I GOT US ON THE CORRECT FREQ; THE CTLR ASKED US ON INITIAL CALL IF WE WERE A LIFE FLT. I KNEW THEN THAT WE HAD FORGOTTEN TO SLOW AS WE WERE TURNED TOWARD THE SHORE. WE IMMEDIATELY SLOWED TO 200 KTS. WHILE BOTH OF US WERE LOOKING FOR THE VFR TFC AND I WAS TRYING TO FIX MY MISTAKE WITH THE RADIO FREQ; WE HAD FORGOTTEN TO SLOW TO 250 KTS PRIOR TO 12 MI FROM THE SHORE AND WERE ABOUT TO FLY UNDER LAX CLASS B AIRSPACE. IN RETROSPECT; THE TIME SAVINGS BY EXCEEDING 250 KTS BELOW 10000 FT MORE THAN 12 MI OFFSHORE ON THAT RTE IS MINIMAL AND NOT WORTH THE ADDED WORKLOAD IN THE HIGH WORKLOAD ENVIRONMENT OF SOUTHERN CALIFORNIA. SUPPLEMENTAL INFO FROM ACN 701792: SELF-INDUCED HURRY-UP OFFENSE MODE. INCREASED WORKLOAD BELOW 10000 FT; FLYING FAST BELOW 10000 FT MORE THAN 12 MI FROM COASTLINE.