Whats the big deal about running a 17" 26" dia wheel/tire combo in the front and a 18" 27" dia wheel/tire combo in the rear?
I understand the speedo recalibrate, but is there really a problem with it, handleing, ride, performance...?

 

Your 3.42 gears will feel like 3.23's with the 27" tires. The added weight and diameter will absorb more energy when the car accelerates, reducing rwHP. Your car will be raised 0.68" higher than stock in the back. The car will definitely have a "rake".