Dave I read this earlier today and have try to think of all of the things that cause the gap to close between the tie rod end and the rim itself. I have the RCAs on my car as well. The one thing that came to mind was the bolts attach the strut to the steering knuckle. The bolts run through the RCAs and it occurred to me that if the shoulder on the bolts didn't have a near interference fit it would allow the strut assembly to move in relation to the steering knuckle potentially causing problems with the toe but also the issue you have. Is it possible that the bolts either do not have a long enough shoulder or are slightly undersized?
Using my crap trig skills I figure that if the bolts were .5 to 1mm under size strut and more importantly the spindle, could at the outer rim edge be swiveling as much as the 15mm you mention?
I've tried to envision where else the gap could change but can't think of how other than at the join between the bottom of the strut and the knuckle.

Tom