erm, the overspeed box allows you to read faster than the default, not write.
The lack of faster burning stratagies will limit the speed you can burn at. If the drive does not have a 16x or 8x startagy for burning a disc then it will not allow burning at that speed, you can only burn at the speeds the firmware has a strategy for.
the 1640/5x drives get around this by making a statagy on the fly for DVD+- R media using solidburn but the 1620 cannot do this as it does not have that feature.
The only way to burn faster than rated speed on a 1620 (or 1640\5x if using DL media) is to buy media that has a faster stratagy built into the firmware, e.g. TY media or swap the stategy around using ala42's program but this is trial and error because you are using power calibration settings for a different disc.