Hm, I overlooked the fact that your wheels are still going to be 18".
When we mention "275/35" or "245/40", we typically mean 20" wheels, and your original size (245/50/
18) is exactly the same as 275/35/
20, so that kind of change upsets nothing. However, 265/35/18 or even 275/35/18 would be tiny compared to your original 245/50/18s, your speedo will be way off, if that setup even works (I never tried even when I had 18" wheels on a different 6th gen Camaro).
I strongly advise that you get tires with more sidewall, 40% at the very least but ideally 45, and Pirelli's sizing doesn't even come close. Overall, as far as I can see, quality 18" all-season tires don't come in the variety you need for a true staggered setup, likely because the intersection of the two buyer segments that want all-season tires and that want very wide sizes is almost zero.
So, I'd consider
- just staying square with better tires in your current size (245/50/18), which the Contis do come in, or
- getting proper summer tires instead of the all-season nonsense; for example, the Michelin PS4S does come in 255/45/18 and 285/40/18, which would be ideal for a staggered 18" setup, and it's a proper 3-season tire that you can use down to 34-35 degrees, dry or wet (not in the snow though)