First of all a 55gr. Bullet from a .223 Remington will not be leaving the muzzle nearly as fast as a 55gr. bullet from a .243 Winchester.
With that said there are other differences as well.
A .223 Remington - 55gr.Bullet has a Sectional Density (ratio of weight vs diameter) of .157 and a .243 Winchester - 55gr. Bullet has a Sectional Density (again a ratio of weight vs diameter) of .133.
A 55gr. Bullet for a .223 Remington is going to be heavier for the caliber as compared to a 55gr. Bullet for a .243 Winchester.
I too love my .243, and use it for Coyotes, but it really tears them up. On the other hand, most are DRT.
Larry