The main reason you want to max out the BEC voltage to 7.4V is to get the servo speed faster, the higher voltage will also improve efficiency and the servo will tend to last longer as well, though a drifter doesn't put much stress on servos to begin with.

That said, with a faster servo you will get faster response which can be good or bad depending on how smooth you are with the steering input.

The gyro gain will likely need to be reduced to account for the faster servo speed so it doesn't "over react" where I suspect that 5% gain will be ideal which was my experience when I used to have a drifter back in the day.

I would try both 6V and 7.4 then decide which feels better for you... you should see that the 7.4 setting will snap into drifts much easier and you'll spend more time playing with the gyro gain settings until you hit that sweet spot.

Good Luck!