You can pole out the gennaker. Probably this will be slower, if the boat is not already at hull speed or close. To work out the speed at different angles versus the extra distance you need the polars or plot them yourself that is speed and angle at different wind strengths. Trying it on my boat there is a good speed gain but the effect cancels out with distance rapidly over 30 degrees. You can get the sail filling better ie not sheltered by the main if you let it forward by letting the halyard out a bit. The best angle then is probably about 20 but you need to work it out in practise depending on wind speed and the boat.