Update: had the wheels/tires road force balanced and got a 4 wheel alignment. They confirmed all four wheels were slightly out of balance, corrected the problem, and their opinion is that the tires are not a problem. They found the driver's front wheel toe-in was out of spec, as was the steering wheel centering. Fixed both issues.
The vibration is much improved, almost gone, but there's still a very, very slight shimmy at highway speeds. Maybe I'm just being too sensitive and it's simply feedback that's normal for the 5. But if any of you are cruising on a smooth highway, steering wheel centered, if you touch a finger on the bottom of the wheel, do you feel a very slight shimmy? If I hold the steering wheel firmly with both hands, I don't feel it. But I do feel it ever so slightly if I'm on a curve at highway speeds.
Also, my brake judder has nearly disappeared! I do still feel a subtle vibration when braking, but to me it feels similar to what I feel when cruising at highway speeds. Maybe it is a rotor issue, and the balancing and alignment helped minimize the judder.
I realize the pads run close to the rotors. Is it possible that deposits on the rotor, or a slightly warped rotor, contact the pads at high speed and cause the vibration? I've never experienced this with any other car I've ever owned, so seems far fetched. The rotor would have to be severely warped to catch the pads when the caliper piston is open, wouldn't it?