I know this is going to be an unpopular answer... or unwanted noise from the peanut gallery... but the idea on consumer cars of getting better performance, or even better mpg for that matter, from a change in tires or wheels or both at the sizes we are all talking about for the purpose of "reducing unsprung weight" is spurious at best.
This concept is a marketing tool for all intents and purposes. In terms of track, straight line or not (and this is a whole other cyclic argument), unsprung weight will simply not start to even be a factor until you start to actually create a vehicle that could take advantage of it, ie, stripped interior, door skins, hood, roof, and fender swapped to plastic or carbon, unstreetable drop, suspension so stiff it is, again, unstreetable, removing ALL accesories and replacing things like pans, covers, oil systems etc., replacing internals with lighter units, block modification for weight, and so on.
With that said, some wheels do "feel" softer than others even with the same shoes, something that was almost universally felt regarding the BBS RC. Forged or cast (regardless of process for each) will make no difference here, weight is weight, and not as critical as some may have you believe. The difference in stock, for example lets say 20" HLTs, in terms of performance, may see an almost immeasurable difference in straight line performance. Any perceived advantage in the twisties will show up due to tires alone, this has more to do with sidewall performance and myriad other very small factors, again, many of which won't be perceptable to the average driver.
With all that being said, forged wheels
tend to have better impact and resonant performance, but this is a gross blanket statement, their are a LOT of really good cast wheels out there (think Germany, Poland, Italy, France, Britain...not china). Lastly, before I get off my soapbox, wheels do not affect mileage except in extreme examples (think chinese chrome ghetto 22s tipping the scales at 40lbs or more), this is primarily HOW you drive and tire choice. You get sticky, your mileage will go down, this is fact. You get hard, performance may suffer, but generally rolling resistance is lessened, hence better mileage (for this example lets say Mich PSA+ vs. PSS). The difference in mileage between a 300utqg and a 500 can certainly be measured, but having had both the older AS pilots can't hold a candle to the PSS. Mom has the new PSAs on her 2013 Gen and they are jawdroppingly good. The drawback at the other end of the spectrum is good luck getting more than 15k out of those Super Sports, ain't gonna happen.
So, bottom line, smaller sidewall = "faster" response, generally a "stiffer" ride (this is purely subjective as we know), mileage determined by rolling resistance and driving habits. Bigger sidewall = less "responsive" but a smoother ride (again, subjective). Forget the unsprung weight sales line and focus on your driving style. You want quicker turn in and less sidewall squirm, bigger
wheel/smaller sidewall. You want better performance from the wheels you have, look at a better brand of tire and lower utqg. You want to increase mileage at the possible expense of performance, then go for a better "touring" or all season tire 500 and over.
/soapbox
I think you may have misunderstood me. Not sure what you mean by 'duh', the big forged wheels mfrs are often the ones babbling about unsprung weight, whether in an 18x10 or a 14x8. Diameter and weight are not related in a linear fashion. I never said bigger wheels couldn't affect mileage and in your case with the 300 I can almost guarantee you were running a diameter greater than stock, which will always have an effect, as would the opposite. Additionally I was referring to unsprung mass having a performance effect withOUT weight reduction, hence, in this example, no change. 
