Mine only shows 1x,2x,2.4x with no disc inserted. I guess it's just the way any particular burner behaves.
Yes, with higher speed media faster options will be available to you. However, don't assume that a 16x burner will be able to burn all 16x media at 16x. NEC in particular are very conservative in this respect & might only give the option of , say, 12x for a particular media. Others, in my case a Benq 1620 will allow 16x on 8x rated Yuden t02 media.