I'm going through this at the moment. Firstly a very light wheel at a reasonable price is TSW. Rotary forged. Rotary forged wheels are very strong. 18's for my 2012 GT at 19.5-20 pounds. Nice.
You'll end up with a top gap in your wheel well which will require lowering which I had planned. Taller tires such as 45 series will likely cause more side wall flex. I would go with a taller timre if I went with 18's.
As I don't track my car at the moment I'm seriously thinking about 20". Great look, minimal lowering needed and the TSW weighs about 23 lbs. Maybe a little extra for the larger tire.
I only have a few styles to chose from as I need Brembo clearance but they have Interlogos with a nice open design.
I wonder how much more the 20" tire will cost compared to the 18"? This is a concern as I'm getting away from the 19's on my GT due to the additional cost of tires. I posted earlier that they are $50.00 more. More than what? More than the cost of a 20" tire that I have on my '06 Saleen. The cost of 18's should be even less but I haven't gone there yet.