Unless you are using Herrie's firmware or the dangerous brothers firmware then you are unlikely to get these 4x discs to write at anything other than the 4x they are rated at.

You could try the 1.07 official firmware if you don't want to use an unofficial one, as this improve the disc compatibility quite a bit.

Even if you could write at 8x I wouldn't really recommend with these discs as they are a bit cheap and nasty.