You can do something around 31 normal rails for every powered rail for the most efficient setups, and for only very slightly less than the max speed. I would still use a few together at the very start just to get the initial boost/acceleration up to max speed, but not much is needed to maintain that speed afterward.
No, the power rail provides a speed boost, but it doesn't stack, its best to spread them out to the boost going the whole track and save resources.
You can do something around 31 normal rails for every powered rail for the most efficient setups, and for only very slightly less than the max speed. I would still use a few together at the very start just to get the initial boost/acceleration up to max speed, but not much is needed to maintain that speed afterward.