4x->8x jump for Z-CLV is typical of Panasonic chipset writers, which the H20L uses. You really cannot compare it to the 4167B because that uses a Renesas chipset. The 6x burning speed is typical of that chipset.
Btw, when burning with the 4167B, 6x is selectable as a burn speed when you use some burning programs (e.g. ImgBurn) but the drive will only burn at 4x when the burn actually starts. The drive does support 6x burning but that only comes into play when the writer itself slows down a burn to 6x (e.g. if you try to burn an 8x disc which the drive doesn’t think will burn well at 8x for some reason). It’s the same issue with 12x burning for 16x DVD-R discs. However, the H20L should allow burning of 16x DVD-R discs at 12x.
LG writers are typically all UDMA2 only, with the exception of the GSA-4166B, which is UDMA4. The 4166B also uses a Panasonic chipset and shares similar features to the H20L. If you want to compare your H20L with another LG writer, that should be the one. Not sure why LG made the H20L UDMA2 though.