Perhaps some of your mods compliment the thicker ARB, were you getting much oversteer before fitting?
This thread was useful in explaining;
http://www.circuitinstruction.co.uk/download-pdf/1-welcome.html?start=8
That's a useful link tbf and it makes perfect sense too. With all the ultra racing parts fitted when absolutely on the limit the car starts to understeer first. But your going rather fast indeed when this starts to happen.......
Gonna see if I can dial some out even further by lowering the rear pressure or slightly increasing the front pressure as the link suggests.