Again, there is no absolute rule to determine what speed is better. The only way is to try and see results.
With your benq 1655 is possible to do scans, so try first to burn a media a notch below the certified speed (for example a 16x media burn it @12x), scan disc and see results.
Regarding standalones, some players are rather picky about media. You can improve compatibility burning +R media setting booktype in ROM: this often improve playability.
To retrieve some informations about burning capabilities of this drive, take a look to its dedicaded thread