I put some 15mm spacers on my Atenza (Mazda6) with stock suspension and stock wheels. The ones I bought were made in Japan and fits only the original Mazda wheels and hub (custom hub sizing) and it wasn't made with weight in mind.
Makes the wheel and tires flush with the fenders, and the wider track definitely improves cornering...but it was about an extra 2.5 lbs per corner and man, can you feel it. Heavier steering wheel, slower acceleration (increased rotational mass), bumps in the road are amplified. I wouldn't be surprised if it shortens the life of your suspension and not just the wheel bearings.
If you do want spacers to make the tire and wheels flush, find the lightest ones you can. I'm not kidding about the extra weight - I recently installed lightweight lugnuts on my wife's Familia (Rays wheels) and the difference was incredible. So promise yourself that the spacers will be temporary until you can buy a nice set of aftermarket lightweight alloy wheels.
For my Atenza, 15mm was as wide as I could go with the stock wheels. Probably similar for the MP3 OEM wheels but they might have different offset from OEM Atenza wheels and fender flare distance from hubs - I'm sure you can find the offset info on this forum. Good luck!