[sd_boxster]

Oh, to answer the original poster's question - running 20's on a Boxster would almost certainly have a negative impact on performance. Presumably they weigh more than the stock wheels, adding more unsprung weight that the Boxster's suspension isn't designed to handle.

I personally can't fathom why anyone would want to raise the center of gravity on a car like a Boxster.

Finally, larger wheels increase rotational inertia - even in the unlikely event that the 20" wheel weighs the same as a 17" wheel, moving some of the mass farther from the axis of rotation will require more energy for acceleration and braking.