even with herrie's new firmware, i cant burn reliably at 8x. i always end up with questionable burns when i do it at 8x. at 6x, nero dvd speed gives me a smooth curve. so in answer to your questions, by burning at 8x, u increase the chance of errors. this is not to say u dont have a good batch that can burn great at 8x. its just that some disks like my datawrite G04s cant manage 8x reliably.
what u should do is burn a 4.3gb disk at 8x, then run it through nero dvd speed. if it passes this, then i think its fair to say you can continue burning at 8x. if u get errors during the scan, then drop down to 6x, see if that passes nero. if that fails then drop down to 4x.
only by testing out YOUR disks can u know whether or not they can burn reliably at 8x.