It does sound as your Z went into a limp mode.
As dkmura mentioned,the Z has a rather sophistcated system
that reads "improper" things that are happening to the car.
Not sure if problem with the TCS is one of them. At least I
have never heard of this happening before. From my experience,
a limp mode action is due to a sensor problem. A diagnostic check
would get the correct information.
Just my opinion,20's on a Z look bad. Just don't work for me.
There is a tolerance of sorts that the system in your car will allow
on tire sizes that differ from stock. I am thinking that with the 20's
you have tires mounted are way over this tolerance. I'm guessing
they are taller than needed.