As it's already been stated, it's your fuel. I would run it a little richer too. If it's pinging on 20 degrees (which is stock timing for a GTi-R) then it's not getting enough fuel either.
I would for the moment (until it's sorted) reduce your boost. Melting a piston will break your heart.
If you must run vortex, then you'll need octane boost to run higher boost on higher timing. Wynns "Race Formula" *cough* is available almost anywhere that sells fuel but it's a hefty $22. It will raise it to about 98-99 ron which is acceptable.
Amaru's gtr dies on 98 ron occaisionally, and when it gets to 102 it runs perfectly. Those 4 points are your friend.