I'm considering going to a set of lightweight 15's.

17's that I've got on there are too much rolling dia. too much unsprung. With no torque they look good on my NA, but don't go too well. I really notice the agility of the car suffering as well. Turn in is sharp but the car feels more sluggish to react to mid corner adjustments than when I had the stock 15's on.

15's seem to be getting tough to find. I've just started a search but I figured if anyone has the inside track it would save time. The upside is, 15's should be dirt cheap now that 18's and up are pretty much the standard now.

 

pd_day (Derek) is selling some stock convertible wheels i think. they are nice and light and a half inch wider than other S5 NA wheels.

other than that 15 inch wheels will be lighter than 17s even if you don't spend the money on the bling forged wheels, although you could definetly go with those to be uber light.
the only problem with NOT going with the expensive brands is you won't find many 15 inch wheels that are wider than 6 inches.
a 15x7 wheel would be perfect contact patch and rotational mass, but yes they are hard to find.

here's one i like, they are good light wheels, not too expensive either:
http://store.racinglab.com/5zfnrawh.html

 

why wouldn't a light 17 or 18 by 8 be a better contact patch than a 15 by 7? you have less side wall, therefore sharper steering responce; wider tread = more surface area = stickier (depending on rubber). If you dish out and get light ones, the advantages far out wiegh any reason I see not to spend the extra money. After all, tires are the only place of traction between the car and road; why not make it as big as possible?

 

ok, ignore the wheels i posted unless you don't want more than a 215 tire to fit.
i forgot they only sell good offsets for that wheel in 17 inch...

as for the contact patch issue, yes you will have a stiffer sidewall with a larger diameter, but if anything the contact patch will more or less be the same, maybe even more as the sidewall flexes.

whether you want really stiff sidewalls or not is a matter of the application, sometimes a softer more predictable sidewall works better.
it may not even matter because there are tires out there that have a stiff sidewall to begin with (Azenis Sports for example)

 

I've got Azenis Sports on my 17's right now. Fantastic tire... I can't say enough about them. Good value too.

 

lightest wide wheels I ahve found are panasports. No one out there makes 15s or 16s wider then 7 inches. Enkei makes some but are available in japan only.

 

There are some lightweight 15's around, but don't forget you'll need tires for them too!
There's not a lot of availability in performance tires sized at or around the original 205/60-15 size. You could use a 215/55-15 or a 225/55-15 to maintain nearly the same rolling circumference. But find the tire / price / size you want before settling on the wheels.
I did recently get a set of 13 lb 15 x 7's 35mm offset (ATS Comp Lite), but as they're for competition, I run a 225/50-15 tire for the smaller diameter gearing advantage.
For the street, you might have better luck with a 16 x 7 35-40 mm offset, and the more commonly available 225/50-16 sized tire. My S5 TII rims & tires are a smidge under 40 lbs - same as the factory 15x6 & 205/60's.
I race on Kuhmo tires & love 'em, their street tires have a lot of sizing, good performance, and are nicely priced!
If you're in GTA, try wheelandtirezone.com in Miss.
Tiretrends.com are good people too.

 

pd_day is also selling these:

https://www.rx7club.com/showthread.php?t=438320

 

oh, i'm also selling some 15x6.5 if you are looking for something cheap:
https://www.rx7club.com/showthread.php?t=435473

weigh about the same as stock 15