From a stock GTV6 wheel I had handy by, I find the OD is 15". I think that converts to 381mm if you want to double check the math.
Some GTV6 owners have added power steering from Milanos, as they felt the steering too laborious when parallel parking and at slow speeds. The diameter of the steering wheel affects the leverage needed. So you may not want to choose a wheel that is much smaller than stock for that reason.