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What sizes/offsets? That can make a big difference. For reference, my 8" ET45 weigh 18.5 lbs (as measured by me). My 10" ET48 weight 19.5 lbs. They are one piece forged and lighter than Fikse, HRE, etc. Don't forget to consider tire weight, etc.
18s. Et is whatever would be the correct fitment for a c2.
I looked it up. Champion made 8" ET45 (same as mine) or 8.5" ET45. For the rear, they made 10" ET48 (mine), 10" ET58, 11" ET68, and 11.5" ET44. They might make more/less sizes/offsets for the 987/997 now. Mine is a Boxster, but Karlooz has 8.5" on the front of his C2. I don't think there is a "fixed" set. Somewhat depends on what tire sizes you want to run.