If you are just looking for part time assist then 250w is plenty. My front hub kit gets my tandem uphill while trailering a dog without having to drop out of the largest ring. I have the swytch you mentioned which btw does have a throttle option though I like the PAS.
I can't even imagine how much the power would be on a single bike not pulling an extra 200 pounds.
If you want full time assist then the larger capacity systems mentioned are probably the way to go.