I've never even thought about foliage width versus rust resistance, but now that you mention it...
Could this have anything to do with diploid vs tetraploid (dips tend to have narrower foliage)? I recall reading somewhere once where some surveys seemed to indicate that dips, on the whole, tended to be more rust resistant than the tets. (Obviously this does not mean every dip is resistant, nor every tet susceptible... but in terms of % of the plants of each ploidy tested, there was a higher % of resistant dips than resistant tets.)
FWIW, I live in zone 9 and I still have some dips here, and they mostly (all?) tend to have narrow leaves. I wouldn't say that they don't do well (unless there is some other issue, such as tree root competition), or do worse than the tets. BEAUTIFUL EDGINGS and TUSCAWILLA TRANQUILITY immediately come to mind - both have narrow foliage, both do quite well here.
