Conventional wisdom for track prep is to replace the pads when they get below 1/3 of original thickness. The last third goes faster; more heat is transferred to the pistons and thus fluid. I'd say put new fronts on. If you don't run with AH on, the rears should be OK> If you do run with "all on", might swap the rears as well.
Keep your take-off's around for contingency spares.
If you're going to be tracking regularly, ... may look into one-piece pads ...
