The 11.75 kit is intended for and proven to fit many 15" wheels. All of them? That's not so easy...the issue is not so much the diameter as it is the caliper to spoke clearance requirement. A 'flat' wheel or one with limited spoke curvature could still be an issue. Most track wheels however are not this way.

You can always check the PDF link on my web page under the Plus 1 kit and see page 3 where you'll find a wheel fit diagram.

BTW....all the offerings of the P2 kit are available for those preferring the smaller size. You can get all the bells and whistles of color and holes etc in either diameter. And less money if you were to do the P2 kit in "optional 11.75 dia" as it would not be the GT rotor and BP20 pad by default in the P1 kit.