Modena Engineering makes a hybrid bolt in gearset with a stronger but helical 5th (or 6th for STI). This si a great way to go to avoid noise when cruising

Staright is much stronger. Surface area is not the problem, angle is. Put torque stright on is better than at an angle. Straight cut holds more power and takes more abuse period. Depending on what you are doing though, an STI 6spd is perfectly fine for mosts goals.