I just replaced my rear tires with the same tires (Michelin Pilot Sport) and am having stability issues on the highway as my 03 G35 sways somewhat at highway speeds. I had the same issue when I first switched from the factory issue tires to a cheaper brand. I had Tire Kingdom replace the cheaper tires with the Michelins which helped but I wound up paying the dealer $200 for a "spindle adjustment" to essentially fix the problem.

Has anyone else encountered a similar issue when replacing tires??

 

Hmmm...I noticed when I went to wider tires than oem's, they seemed to grab the roads imperfections more and had to correct with steering more often. Felt it in the back too.
Then soon after, got an alignment which improved that condition a lot. My alignment was off.

New tires in general take a little while to scratch in. My KDW2's took about 300mi to finaly feel hooked up.
My Sportmaxx on the other hand took much less time to hook up from being stickers.

Your hwy speed swaying feel could be alignment issues with combo of tires not scratched in yet
Don't go too cheap on tires. Make sure they're y or z rated.

 

http://www.tirerack.com/tires/tirete...e.jsp?techid=5

"Tires are comprised of many layers of rubber, steel and fabric. Due to these different components, your new tires require a "break-in" period to ensure that they deliver their normal ride quality and maximum performance..."

 

^+ you just need to break them in...

 

These cars greatly benefit from a 4 wheel alignment after any tire or wheel changes.

 

You should get an alignment anytime you replace tires.