FLC ON A B737 AT MIA WERE INSTRUCTED TO TAXI OFF THE RWY AFTER ASKING FOR EXTRA SEPARATION BEHIND DEPARTING B747.

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WE WERE HOLDING SHORT OF RWY 9L AT MIA. TWR CLRED A B747-400 TO TAKE OFF FROM RWY 9L. TWR THEN CLRED US INTO POS AND HOLD ON RWY 9L. THEY THEN CLRED US TO TAKE OFF BEHIND THE B747./ BOTH THE CAPT AND I HACKED OUR CLOCKS WHEN THE B747 BEGAN HIS TKOF ROLL. WHEN TWR GAVE US CLRNC TO TAKE OFF; WE WERE ONLY 1 MIN BEHIND THE B747. I TOLD TWR THAT WE NEEDED MORE TIME FOR SEPARATION BEHIND THE HVY. HE TOLD US TO CLR THE RWY IN AN ANGRY MANNER AND TOLD US THAT IF WE NEEDED MORE THAN NORMAL SEPARATION THAT WE SHOULD LET US KNOW. THEY THEN TOLD US TO CLR THE RWY AND TAXI INTO POS AND HOLD ON RWY 12. THIS REQUIRED A CONFIGN CHANGE; AND PWR CHANGE; AND MCP CHANGE. I HAD THE NEW TKOF PWR SETTING FOR THAT RWY AND WHILE THE CAPT WAS TAXIING INTO POS; WE BRIEFED THE NEW DATA; FLAP SETTING; AND TKOF BRIEFING. THEY THEN CLRED US TO TAKE OFF. REST OF FLT WAS RELATIVELY UNEVENTFUL. TWR HAD CLRED SOMEONE TO LAND ON RWY 9L WHILE WE WERE IN POS ON RWY 9L AND THE B747 HAD DELAYED HIS TKOF ROLL ABOUT 30 SECONDS AFTER TWR CLRED HIM TO TAKE OFF. CTLRS AT BUSY ARPTS SUCH AS MIA SEEM TO BE TRYING HARDER AND HARDER TO PUSH THE DEP AND ARR NUMBERS UP. WE NEED TO KNOW THE REGS AND NOT GET PUSHED INTO A CORNER WHERE WE MAKE BAD DECISIONS JUST BECAUSE WE BELIEVE THE TWR CTLRS WILL PUT US IN THE BACK OF THE LINE. WHILE THIS INCIDENT WAS HANDLED APPROPRIATELY; IT WAS STILL A CASE WHERE CTLRS ARE TRYING TO PUT US PLTS INTO POS THAT ARE ON THE EDGE OF SAFETY.