"Unbalanceable" tires is often due to excessive road-force variation. Most shops don't even have a machine to test for it. High road force variation essentially feels like lumpy tires and can cause noise, vibrations, and steering problems, all ranging in severity.
Also, the axles could be out of balance.
As for rotors not commonly warping and it being a build-up is not really accurate. Rotors will glaze way before pad material sticks to them. Organic pads and Asbestos pads are a thing of the past. A decent set of Raybestos, Bendix, or Wagner performance pads will produce minimal dust, build-up, and wear. Think about it logically: a high spot on the rotor or pad will not cause the side-to-side motion of a warped rotor. Pad material does not build up on one spot from normal use with modern pads.
Chinese rotors warp like crazy. They are cheap and thin and the quality is inconsistent (hard spots). If someone opted for the $18 rotors while doing brakes, I garuntee warpage in no time on your SHO. Excellent rotors are generally Italian (Brembo) or German and are quite expensive. Considering the mileage they last, I think it's well worth it. Cheaper rotors such as Moroso can be had at decent prices, but a set of nice ceramics will warp and blue those babies instantly.
You can take apart the calipers and check for seizing or resistant slider pins, missing or broken shims, and worn or torn dust boots and piston seals. If any of these are present, the caliper may not be opening well. As the rotor turns through the caliper and it has the slightest warp, it will graze the caliper causing noise or vibrations.
But most likely at this point, you may be missing a weight on an axle.
BTW, a cut rotor is not always a true rotor. It must be mic'ed, checked for runout with a dial indicator, and be within specs. Most people that cut rotors wont bother to take the time to check runout before and after the rotor is cut. I know i didn't until I found out that I couldn't true a problem rotor by cutting it.