You don't know anyone with 16" rims for whom you can try?
18" to 16" you lose an inch radius, that being from the hub to the
rim. So, measure the caliper to the inside of your rim. If you have
more than an inch of space between the rim and the caliper, the 16s
will fit.
They seem to do the same thing everyone else does... For example, my
'12 Jetta came with 288MM rotors. All I needed to do to upgrade to the
312MM was buy the caliper brackets and 312MM rotors. From what
MrAwesome said, that seems to be the same thing from V6 to GT, they
use the same caliper. The GT has a bigger rotors, so the caliper bracket
relocates the caliper outward. Also, the PAD surface is the same, same
with my VW. The rotor, being larger simply has a space between the
hat and pad surface, where as the V6 rotor does not.
Example:
288MM ROTOR:
312MM ROTOR:
Look at that gap from the HAT to the PAD surface, compared tot he 288MM rotor.
That is kind of how they cheat with bigger factory brakes. Look at the difference
too, from the wheel:
288MM -vs 312MM:
Look how much farther out the caliper was pushed out, with just the brackets.
You can also see the space there between that spoke, from the hat to the rotor
that you do not see in the 28MM rotor.
BRACKETS:
Left bracket is for 312MM rotors, the right one was the 288MM. That's all they do
in some cases to make bigger brakes. IT dissipated heat faster, but the pad surface
remains the same.