The busa was 50 to 100m ahead and as I cut through the apex my peg feelers were dragging at about 100kmh and I shifted to fourth and opened the throttle to close the gap.
You were at the Apex and correct me if I am wrong you downshifted to fourth and opened the throttle?
most possible cause of losing traction was your engine speed when you downshifted and opened the big boy up, higher revs faster turning rear wheel, in a lean angle
makes the old arse pucker does it not
engine speed did not match road speed, as the great Jackie Stewart said never get back on the throttle coming out of a turn until you know you will not let it off
Last but not least slowdown and enter the turn with a speed that will let you increase throughout the turn---a smooth steady rider will outperform a fast erratic rider any day(not saying you are a bad rider)