the only time's I ever shift are while braking, and if I need more power I'll sometimes upshift during a drift, but that most often takes the rev's out of the power band of the car, even with a pretty broad power band, so 99% of the time I just stick to the gear I am in once I am in a drift, and adjust my line to compensate for the lower speeds so I dont go flying into the wall or something.
And actualy, by holding the gas pedal at 50% while shifting, you're forcing the game to power shift, its a great way to maintain speed, but I usually dont do it, it happens by accident sometimes tho, lol.