C206 PLT AND A GULFSTREAM BUSINESS JET HAD LESS THAN COMFORTABLE SEPARATION IN VNY CLASS D.

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WE WERE APCH VNY FROM THE N TO LAND; AND HAD CHKED IN WITH THE LCL CTLR; ADVISING OF OUR POS AND ALT. HE ADVISED US TO FLY DOWNWIND FOR RWY 34L. THE CTLR THEN CLRED A GULFSTREAM FOR TKOF. WE (THERE WERE 2 QUALIFIED PLTS IN THE AIRPLANE) DID NOT HEAR AN ALT ASSIGNMENT OR HDG FOR THE GULFSTREAM; BUT AFTER HE TOOK OFF; TWR ADVISED US OF THE GULFSTREAM; AND WE ADVISED HE WAS IN SIGHT; AND THE PLT-RATED PAX KEPT AN EYE ON THE GULFSTREAM WHILE I CONTINUED DSCNT AND SET UP FOR LNDG. AT APPROX 2500 FT MSL AND 2.5 NM FROM THE ARPT; THE GULFSTREAM TURNED TOWARDS US. WE EXPECTED HIM TO CONTINUE HIS CLB; BUT HE WAS NEARLY AT OUR ALT; SO I GREATLY INCREASED THE RATE OF DSCNT. HE PASSED JUST OVER AND BEHIND US. I WAS ABOVE TFC PATTERN ALT AT THE TIME (TPA IS 1800 FT AT VNY); BUT WAS ALSO A BIT FARTHER OUT THAN USUAL FOR TPA. WE ADVISED THE TWR OF THE INCIDENT; AND HE; THE CTLR; INDICATED SURPRISE AND STATED THAT THEY USUALLY CLB FASTER THAN THAT. THIS COULD HAVE BEEN PREVENTED BY THE GULFSTREAM PLT BEING MADE AWARE OF OUR POS AND ALT AS RPTED TO THE TWR. THE TWR ONLY ASKED IF WE HAD THE GULFSTREAM IN SIGHT; NOT VICE-VERSA. IT WAS PARTICULARLY IMPORTANT THIS TIME; AS HIS INITIAL HDG AFTER TKOF WAS APPARENTLY RIGHT IN OUR DIRECTION. HAD WE HEARD THAT HDG ASSIGNMENT FOR THE GULFSTREAM; WE PROBABLY WOULD HAVE DUCKED DOWN MUCH MORE QUICKLY. IT IS UNKNOWN IF THE GULFSTREAM HAD TCASII OR IF HE SAW US; BUT IT WAS WAY TOO CLOSE FOR OUR COMFORT.