The 8.5GB on the cover of the discs takes GB to mean 1000MB when the traditional term, now called Gibibyte, or GiB, is 1024MB. Therefore, the 8.5GB on your DVD-R is infact 8500MB and therefore 7.957GiB, and if Nero takes a Gigabyte to be 1024MB then that explains it.
I don't know about DVD overburn, I've never actually done it.
Edit: Wrong number.