If the outer diameter is less with the same pad then you will have less braking surface area, so therefore less braking. Also, are the two piece rotors the same offset? If you want improved rotors just go with a two piece 14" that fits the stock calipers.
Most upgrade to the 4pot stop tech or 8 pot Brembos and don't stick with the stock calipers.

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2003 X5 4.6is Estoril Blue
2005 Infiniti G35X
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