Hi, and sorry for my average english. I had a similar issue on my own IS which I bought last year. The important thing is that it is a 2013 year model, which means it is one of the first model built. I had to battle a bit with the dealer to make him accept fixing the issue. I had a one year warranty and I paid nothing. There was a recall on the first IS 2013 models, I may be able to find the recall ID... Anyway, my car feels now like a normal car while it was more like you described prior to the fix. Like a boat at high speed. It had nothing to do with the tires or with the battery pack in the boot. Found the Technical Service Bulletin : Notchy steering feeling CP-0044L-0314_IR
