The local review of the 4166B used the 1.02 firmware (for the 4166B) firmware which SaboCD mentioned. So by looking a the DL part of the review we can see how well the drive handles DL media :
Verbatim 2.4x DVD+R DL seems pretty good. With the 8x version, the second layer is still burned at only 4x. However, burn quality looks better. Ignore the burn quality with the Ritek DL disc. Most drives have problems with these discs, so it is the discs that are at fault not the drive.