Ted, the answers to those questions are beyond my current and future understanding of all the dynamic variables .
So if your main concerns lie in not upsetting some magical balance then stock/stock may be the safest bet. However, if you are willing to compromise some things to get others, then by all mean go for it but don't get caught in the cogs of the hair-splitting factory in the name of "better performance".
16's, 17's, 18's are all reasonable choices since tires with the correct widths and rolling radii are available (thank you Porsche). For example; if your main concern is weight and MOI then do 16's, if you want short sidewalls and bling-bling do 18's, if the wheel you like only comes in the right style, width and offsets in 17's then do those, but don't justify your choice with one or two marginal reasons when significant variables aren't even being considered.
In short, I don't know

BTW, 18's with stock ride height = 4x4, just ask William, he's got two examples at home (S2000 and Mustang) j/k William

 

I think everyone and bin Laden have seen William's 4x4 red S2000 on this board.

 

In summary. If you plus size, you'll want to lower the car. The old rule of thumb is that the space in the wheel well should be proportional to the height of the tire sidewall. Sidewall flex can be mitigated with increased tire pressure. The more you lower the car, the less negative camber you can dial in. Even if you go with a lighter wheel, if the wheel is larger, more of the weight is pushed to the outside of the wheel increasing MOI. Yeah this is splitting hairs...

Pick a sets of wheels that you like, and stick with it. If you can't decide, perhaps another set of stock wheels are in your future.