Welcome to CD Freaks.
Some say yes that burning slow does improve burns. However most here would say NO.
for 4x & 8x rated media burn at the rated speed. For 16x media most probablyburn at 12x with very good results, although the difference between 12x & 16x burning quality is likely to be very small.
With some of the 18x & 20x burners very good results are being achieved with quality 16x media burned at these higher speeds.