You may find this interesting:
First of all, check the tire pressure; cars are often shipped with extremely high tire pressure to avoid getting a flat spot on the tire. Then get the wheels balanced. If the problem persists, you may have a Radial Force Variation (RFV) problem. You can see what Hyundai tells their dealers about wheel shimmy
here. Click on "NVH", then ''Tire/Wheel NVH". They know RFV is a problem and tell their dealers how to fix it using a Hunter balancing machine. Considering that Hyundai now owns Kia, perhaps the Kia dealer has access to this information.
Anyway, you may want to print this out and take it to your dealer. Have them balance the tires on the Hunter machine as mentioned at the link I provided. Let us know what they say; I'm considering buying a Spec5 myself.
On one test drive, I felt the same vibration at 70 mph. It was very minor, but clearly there.