I found a good deal of a set of 245/45/18 pilots and i was wondering what would happen if i put the same tire size all around the car... would it effect the handling really bad

 

Are your rims staggered (8.5/9.0)? I think your plan will affect handling moderately, especially if you're coming from a staggered setup. As far as the safety factor, I would be a little bit hesitant as the coupe suspension/dynamics are built to run a staggered wheel setup.

If you're working with a reputiable tire shop, they should steer (no pun intended) you in the right direction.