Chrysler 300C Hemi Touring Tyres

I bought this car with 5,000 miles on it and the tyres already showed cracking along the grooves. At under 17,000 miles they are now approaching the wear markers - front and rear almost equally. The front 245/40 x 20s are wearing at the outside corners and feathering slightly so I suspect wheel alignment. The wear on the rear 275/35 x 20s is disappointing as I do not feel I have driven these Dunlops hard. I cannot say much about the tyre noise except that my other wheels, carrying 225/50 x 18 Pirellis are quieter, but the comparison is not valid since they are smaller than the Dunlops.
My rear Dunlops are 275s on a 9" rim and they protect the rim. The fronts, however, are 245 on the same 9" rim and the rims are wider than the tyres so don't get any protection. I feel that the 245s are undersized for the fat 9" rim, and that the tyre protection on the Dunlops would otherwise be adequate.
I'm going to try cheaper tyres next.