You replaced the rotors and calipers, right? Maybe put the old parts back on or buy some cheapo OEM style rotors to see if that changes things.
Also, small alignment changes can change the pre-load steady state forces on the wheels. They might be happier with a little toe in or out to put some load on them while driving straight, or may prefer more or less camber and/or caster.
My brother had a car that had un-fixable front end shimmy when he used wider aftermarket wheels. The car was an old econobox and ran just fine with stock parts, but when he put on wider wheels the geometry and force balances all changed to where the tires would wobble badly at several different speeds. Nothing can possibly fix that except radically changing the geometry to make it stable again. Maybe a different offset, or different weight wheels/tires. Maybe adding a 10 lb weight to the steering arms would have done it. Regardless, something made it dynamically unstable.
And that's the point I'm making with swapping parts. If you just swap existing parts for newer stock-shape/weight parts with better bushings, you're not changing the inherent dynamic instability. You're just dampening it out with stiffer bushings. You need to change the dynamic system with geometry (alignment or wheel size/offset) or weight. Geometry changes can eliminate the vibration, weight changes will probably only shift the vibration issue to a different speed range.
Lighter wheels and even lighter or heavier brake rotors/calipers will affect that. Same goes for alignment changes or wheel size/offset changes. I suspect that the entire front end was only marginally stable to begin with and they pushed it out the door for sale without solving it in the first place. They just picked the best of what they had and hoped it would be good enough that people wouldn't notice.
It could be *anything*, even those stupid dangly weights in the back of the car that many people remove, if it changes the dynamic harmonic vibrations in parts of the car. A tiny vibration in one spot can be amplified through suspension components and if they end up focused on the steering components, you bet we'll feel them through the steering wheel.
Heck, you've seen this yourself while pushing around shopping carts. Most go just fine but sometimes one of the front castered wheels on a shopping cart will freak out and vibrate like crazy, right? Why does only that one cart do it and not other identical carts? Put a 24 pak of soda in the cart, and suddenly that front wheel doesn't do that anymore. Why did just increasing the load stop the vibration? Same thing with our cars. And GM doesn't seem interested in engineering a solution or sharing what they know about it, so EF them and the horse they rode in on.