Your engine has a 9.5 to 1 compression and technically it does not
change. But, with more air forced in by the turbo the compression "seen"
in the enigine is higher (turbo charged engines could be described
as a variable rate compression engine). So, I think you would have to
run as high an octane as possible since the chip is probably pushing
the design limits.

I have a 98 T5M and on mid-grade (89) I have heard it ping on a hot day.
This is a stock engine. It only pings once if this happens but it is
something you don't want to hear. I normally use 91 octane (highest
available in California) and sometimes wonder if I chipped my car
if 91 would even be high enough.