I dont know whats the problem with 15 x 10's. I also read that when I got the kit, and wondered why. i cant see how it would have to do anything with the tires rubbing in the spring perch or something like that. I mean if you use the right backsspacing then what does it matter. I put on 15 x 8's but after reading that, I almost thought of going to 10's just to see why.
sorry this doesnt relly help you but I was just curious for an answer too.
Maybe trip will chime in, I know he runs this kit.