It makes a noticeable difference in handling and acceleration.
To address the original question, it is possible to get wheels in the 16 -17 lb range for very reasonable prices. Sub 16 pounds and either you are looking at exotic materials like magnesium, or you are looking at a very fragile wheel, really a track only application. Even at 16 pounds you are going to suffer more visits to the wheel shop than you will with the stock wheels.
My favorite wheels for low weight, resonable cost, quality construction, quite durable and the fact the name says abit of their heritage, RPF1 by Enkei, F1 due to the same style albiet vastly different size they run on the F1 circuit
I had the 17x8 at only 15.6 lbs, drove them very hard on race tires, hit a curb once, slight bend in the rim, hit the curb hard enough to tweak the heck out of the Tein pillow ball mount camber plate, just let the air out and taped it up, used my 8lb dead blow hammer to put it back in shape. Another dozen races, a few thousand miles on the street(or race tires just as good as new, could not even find the damage again when I tried.
That is a great wheel. Enkie is going to help me out again on the tC but not many to choose from that are this good, may run the same wheels again.
And yes, the weight difference is very dramtic, once you have something like this you will never want heavy wheels again, most tuner wheels are total BS crap that ruins performance in the name of "style", god I hate that crap.
The only reason I got the 5Zigen GN+ is because I wanted to run 245s on a 8.5" rim. They are a tad heavy at 18.75 lbs in a +30 offset (mine are +48, same as WRX), but not nearly as bad as most. If they made the FN01RC in 8.5 wide with a 5 x 100 PCD, I'd've bought them in a heartbeat. Closest I could get is 8.0...
I fully understand buddy, I am seriously interested in the same wheels you have except the "black chrome" or "hyper silver" whatever they call their version of the stock looking wheel color. I would take the extra weight for extra rubber as well, sometimes tradeoffs are required to get what you want.
Since I can get Enkei's for a killer deal I am very limited to what I can run so may end up with the RPF1 again, really love them but not quite the right style for this car. Would be better in the chrome look paint, hyper whatever, but not made in our size.
I plotted out the stock and all other wheels available for offset, rim width, weight and only a few will do it for this car. I am almost forced into 18's which I really do not want to go with, I would even go down to 16 or 15 if anything was available that would be wide and light enough
Seems we both understand real performance when it comes to wheels and tires
Stock wheels weigh in at about 20 lbs and a few ounces more. That's the most "accurate" information I've seen. It's still heresay as it's from a thread with a guy saying he measured the unmounted wheel sans tires on a postal scale and I'm inclined to believe him. His exact reading was 20 lbs 3 oz.