[Quickurt]

You may also want to be aware of the outside temps. He is in Canada and some of the ultra high performance summer tires are very unfriendly to cool temps. They can flatspot until good and warm and I've also heard of some other roundness and wobbling issues with undertemp high performance tires. I took a set of Bridgestone S03's to Ohio in December, once. No snow, but I think I was half way back to Florida before the flat spots finally went away.
A shock issue could explain the ride harsh/ride okay instability, but I've never heard of shocks causing a shimmy situation. Alignment can and uneven tire wear and/or scalloping from poor alignment can definitely cause speed related tire vibrations.