POTENTIAL CONFLICT IN PROX OF APCH RTE. INFLT WAKE TURB ENCOUNTER.

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AS PLT OF TRINIDAD TB21; I WAS CLRED FOR A DOWNWIND DEP FROM BFI; RWY 13R. AFTER TURNING DOWNWIND FOLLOWING TKOF; I ENCOUNTERED WHAT I ASSUMED TO BE WAKE TURB FROM A TWIN ENG HVY ON FINAL TO SEA-TAC; WHICH I COULD SEE TO MY PORT; AT OR BELOW MY ALT APPROX 1/2 MI TO THE S. MY ALT AT THE TIME OF THE WAKE TURB ENCOUNTER WAS APPROX 1000 FT. MY ACFT ROLLED TO STARBOARD; AND EXPERIENCED A LIFTING ACTION WHICH BROUGHT MY ALT TO 1100-1300 FT. I AM UNCLR OF THE PRECISE ALT INASMUCH AS MY MAIN CONCERN WAS RIGHTING MY ACFT AND MAINTAINING LEVEL FLT. I CORRECTED FOR THE ROLL; AND SAW TO MY STARBOARD A FREIGHTER JET ON FINAL TO SEA TAC WITH ANOTHER JET BEHIND HIM A COUPLE OF MI. AT THE TIME I SAW THE FREIGHTER JET; IT APPEARED TO ME THAT HE AND I WERE CLOSE TO THE SAME ALT. I SPOKE TO BFI TWR CHIEF; AND SHE INFORMED ME THAT THEY HAD HEARD FROM THE FREIGHTER PLT WHO WANTED TO KNOW IF DOWNWIND DEPS FROM BFI WHEN INCOMING HEAVIES TO SEA TAC ARE OVERFLYING BFI. INFORMED THAT THIS WAS NORMAL; THE FREIGHTER PLT INDICATED THAT HE THOUGHT HE WAS ABOUT 1400 FT; AND THAT I WAS 1100-1200 FT. HE TOOK EVASIVE ACTION WITH A SLIGHT TURN TO PORT. I DID NOT TAKE ANY EVASIVE ACTION; IN THAT IN MY JUDGEMENT; NONE WAS REQUIRED. IN ANY CASE; MY FLT PATH WAS SUCH THAT I WAS DIVERGING FROM HIS AS FAST AS WAS POSSIBLE ANYWAY. I AM QUITE SURE THAT I BROKE THE 1100 FT CLASS B AIRSPACE THAT OVERLIES THE S END OF BFI DURING THE WAKE TURB ENCOUNTER. ON REFLECTION; THE FOLLOWING PROCEDURAL CHANGES SHOULD BE CONSIDERED BY ATC AT BFI. WHEN VISUAL APCHS TO SEA TAC FROM THE N ARE ACTIVE; DOWNWIND DEPS FROM RWY 13R AT BFI SHOULD NOT BE AUTHORIZED. I REALIZE THAT IT IS ALWAYS THE PIC WHO MUST TAKE RESPONSIBILITY FOR WHETHER A PARTICULAR DEP IS APPROPRIATE; BUT TO PLTS UNFAMILIAR WITH HOW THE TFC FLOW FROM AN ADJACENT CLASS B AIRSPACE AFFECT OPS FROM THE ARPT THEY ARE LNDG OR DEPARTING FROM; HELP FROM ATC MAY BE VITAL FOR FULLY INFORMED DECISIONS. SINCE HEAVIES INBOUND TO SEA TAC ARE PASSING WITHIN 300 FT OR SO OF DOWNWIND RWY 13R TFC; WAKE TURB ENCOUNTERS ARE UNAVOIDABLE. I TAXIED FROM FBO AT BFI; AND WAS CLRED FOR A MID-FIELD DEP FROM RWY 13R. SUCH MID-FIELD DEPS RESULT IN GA ACFT TURNING DOWNWIND AT THE PLACE WHERE OVERFLYING HEAVIES INBOUND TO SEA TAC ARE THE CLOSEST TO THE TPA AT BFI. WHILE MID-FIELD DEPS FROM RWY 13R ARE THE MOST CONVENIENT WHEN YOU START OUT AT FBO THESE COUPLED WITH DOWNWIND DEPS ARE DANGEROUS. IN THE FUTURE; I WILL REFUSE SUCH CLRNCS; AND OPT FOR THE RWY 13L DOWNWIND DEP; WHICH PUTS GA ACFT ON A SAFE DEP WHICH AVOIDS HEAVIES INBOUND TO SEA TAC. I RECEIVED NO WAKE TURB WARNINGS ON MY TKOF CLRNC FROM BFI. SUCH WARNINGS SHOULD BE SOP; AS THEY MAY CAUSE PLTS UNFAMILIAR WITH THE DYNAMICS OF THE INTERACTION BTWN THE 2 ARPTS (LIKE ME) TO MODIFY THEIR DEP REQUEST.