This is very strange, I've never tried overburning a DVD+RW and just now I just made a overburned DVD+RW using Nero 6.6.0.16 and Plextor PX-716A. I remember awhile back that DVD+RWs cannot be overburned. How is this possible that I can overburn a DVD+RW?

Where did you hear/read that +RWs can't be overburned?
I have never overburned +RW media, but because you succeeded I assume it is posible with the Plextors that can overburn.

How large was the overburn? Is the disk fully readable?

There's no reason why overburning wouldn't work. However, speed increases aren't possible.

I actually tried overburning a DVD+RW in the past and Nero would give me a message saying something like "there isn't enough room on this disc...." I guess with new updates from Nero along the way it allowed DVD+RWs to overburn. The overburn was 4567MB (4600MB Max) and the disc is 100% readable.

I burned a movie onto the disc, seeing there is no point in putting it on non-rewritablable media because I only watch it once, and the movie was free of errors from beginning to end. I also ran Nero's Scandisc just for the heck of it and the result was 100%.