Your post has been split from the 16P9S@Newegg thread in the Bargain Basement and has been moved to the LiteOn forum.
This is normal for all types of drives. Max speed is always reached at the outer edges of the disc. For a single layer disc, the outer edges represent the end of the disc. For a dual layer disc, the outer edges usually represent the middle fo the disc (depends on the track arrangement, but in most arrangements, the outer edges represent the middle of the disc, hence the deceleration).
The 166S can reach a max. of only 10x when ripping dual-layer discs. Its default speed is 8x. There are many variations of the 166S patched firmware, each with a different set of max. speeds. Whoever flashed your drive probably flashed it with a firmware that has dual layer set to 8x and not 10x.
Since you had originally posted in a 16P9S thread, are you considering getting a 16P9S? The 16P9S is able to rip dual layer faster (reported stable speeds of up to 14x and possibly even 16x if you have a very good unit with very good media)