I don't trust those swizzle stick plugs or any other kind at higher speeds. That means I wouldn't go over 80 if I had a plugged tire. That said, I've never had an issue with any plugged tire. I don't know what the law says regarding the use of plugs here in California, but I do see the regular swizzle sticks at the parts stores.
A few years ago my left rear tire ("Bad"Year RSA, not plugged) disintegrated going down the freeway at about 75 mph. There was lots of droning noise, but absolutely no sign of a tire problem. No pulling, no shudder, nothing. By the time I could pull over and stop, the completely deflated tire had begun spinning on the rim and was smoking. There was never ANY indication other than noise that anything was wrong. A deaf driver would have kept going merrily, smoking, catching fire, burning... While this is a testament to the exceptional design of my car's multi-link suspension, I believe a tire pressure monitoring system should be mandatory on the A4. I had a rear tire blow out going 125mph in my Scirocco once, and while it was pretty scary, there was no doubt what had happened.