I'm having issues with my internal NEC 3550A and LG GSA-H10N. I've updated the firmware for each to the most currrent (from cdfreaks firmware, JL11 for the LG, and MAD DOG MD 1.F2 for the NEC). But at 16x burn, I'm only getting up to 8x on the first 35 sec of burning then starts to drop down and settle at about 5x. I've used Maxell, Fuji, Imation, Sony, and Philips 16x media. And all are testing at that speed.
I've tested the burn speed on these drives using Toast 8, and MBBench (Mac). My older Mac G4 had the same NEC 3550A, and it burned at a much faster rate. That's how I first noticed the slower speed on these drives.
Using Toast to test time on burning. On the older G4 it would take about 3.5 - 4 min to burn a full 4GB DVD at 16x. The burners on my new G4 says it will take about 4 min, but actually takes about 10 min.
But ripping a full 4GB DVD, burning CD-R at 48x, and reading DVDs is all within normal parameters. And I don't think I've ever had a bad burn (maybe one) under "16x" burn option.