Widest tire that will safely fit on stock 18x8 wheel?

Hello all, my car currently has the stock all season Pirellis with a 235 width and they are starting to be overworked (loss of traction) since adding more power to the car. I figure just switching out the rear for a high performance tire at the same width would help this issue. However, I would like to have a wider tire as well. What is the widest I can go on the 18x8 wheel while not looking too bulgy or being a safety hazard? Any specific tir brands I should look into or avoid? Also, if I switch out the rear, do I need to do the front tires as well?

Thank you!

on a 8" 245 is my max sizing I say.