Basically, a smaller wheel diameter is beneficial because it has less mass (unsprung weight) and less rotational inertia (which is more than just the rest mass difference; there's a multiplier in there for wheel radius for the mass at the outer rim), all other things being equal.
In the net you basically always want the smallest wheel/tire diameter possible. The reason we don't run e.g. 15" wheels on this car (well, aside from it looking stupid) is because the other balancing factor is being able to fit the brakes in the wheel. Even if you custom-built a brake system for your 370 that could fit in a 15" wheel, it probably would not have enough mass or surface area in the rotors to take the heat/abuse. A 3000lb+ car needs fairly beefy brakes.
Given that most 18" wheels fit over the stock/sport brake systems, the step from 19 -> 18 is a pretty obvious, easy win.
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