I had the same problem in the past (IIRC after I installed DVD Region Killer and/or Alcohol). However, I didn’t remove the DRK filter because I thought it was hardware problem (I blamed my mobo).
At that time my BenQ DW1640 only has burst rate of 17~18MB, 20A4P 23~24MB, Litey could do 16X TRT, BenQ would have problems going more than 12X, everything just like yours.
Recently I did a clean reinstallation of Windows XP, had no Alcohol/DRK installed. BenQ DW1640 having burst rate of 23~24MB, 20A4P 9P57 does 37MB, things get back to what they should
Your sammy should have burst rate of more than 30MB normally.
So, this would be a problem caused by software, you could check the upper/lower filter with this utility and remove the third party filter(s).