I too have seen this drop in burn speed to 4x after about 4 - 5 burns at 8x. I found that flashing back to BSGB and burning a couple of sessions on a multi session disk brings it back to 8x speed each time.
I use Sony D11 8X +R and Moser Baer 8x +R media (these are the only ones available locally) and it is randomly either one if these media which will drop to 4x speed. I have never seen both media dropping their speed to 4x at the same time.
I have not had any success with overspeeding with QSuite2 or MCSE so I have given up trying - my BSLB firmware is now used at its default state.