ASTRA CREW WAS OPERATING THE ACFT TOO HIGH FOR PROPER BUFFET MARGIN.

Narrative

DURING CRUISE AT FL450 ENRTE FROM EUGENE; OR; TO SOUTH BEND; IN; IN CONVECTIVE ACTIVITY; A DSCNT TO FL410 WAS INITIATED DUE TO AN INCREASED DECK ANGLE; DECREASED AIRSPD; AND SUBSTANTIAL DEPRECIATION IN PERFORMANCE INDICATED BY THE ANGLE OF ATTACK AND WAS FOLLOWED BY A VERY NOTICEABLE TAIL BUFFET. THIS WAS DUE TO AN AGGRESSIVE DOWNDRAFT AT THE TIME; NOT A MECHANICAL WHICH WAS SUGGESTED TO THE NTSB BY ATC. I FELT A STALL COULD HAPPEN WITH LITTLE OR NOT NOTICE DUE TO THE FLT CONDITIONS WE WERE EXPERIENCING. WE REQUESTED A DSCNT TO A LOWER ATL 3 TIMES FROM ATC. THE FIRST TIME ATC TOLD US TO STAND BY. IN THE SECOND ATTEMPT WE TOLD ATC WE WERE EXPERIENCING TAIL BUFFET; AND NEEDED LOWER IMMEDIATELY. ATC TOLD US AGAIN TO STAND BY AND THEY WOULD TRY AND COORDINATE IT FIRST. DUE TO MY SIT WORSENING; AND AFTER SEEING NO CONFLICTING TFC ON THE TCASII; I FELT I HAD NO OTHER CHOICE BUT TO ADVISE ATC I WAS DSNDING OUT OF FL450 FOR LOWER. ATC THEN IMMEDIATELY CLRED US TO FL410. I DID WHAT I HAD TO DO BECAUSE ATC WAS SO OBTUSE TO A GROWING CRITICAL SIT. IT TAKES A COORDINATED EFFORT BTWN ATC AND THE ACFT THEY CTL TO KEEP TFC MOVING SAFELY. AFTER ALL; SAFETY IS PARAMOUNT. OUR ENVIRONMENT CAN BE VERY HOSTILE AND MOST UNFORGIVING IF NECESSARY ATTN IS NOT GIVEN TO DETAIL.