As others have said, the main issue is the time it takes for HID to warm up. Since they won't be at full power for 20 seconds or so, when you flip on the highs, you have almost light, and you are presumably already at speed. Definitely do the quad-beam mod, which allows the low beams to stay on when the highs are on. Even without HID high beams, the quad-beam mod is excellent.
As for the flash-to-pass, its not that they won't work, but since they are not instant on, if you flash at someone, all that really happens is the high beams flicker.