Returns are 20%-25%, with my calculations its 24% but it can swing. But it's a huge profit, but I would only do it, when you have enough steempower yourself.
For powerdown, send a memo (0.001 steem) to minnowbooster, it will return an error, but they use a script to catch the memo and do the powerdown once in a certain time interval.
You can also just ask minnowbooster support, and they will send a message for powerdown to the admins.
Again, this is imo a longterm investment. Dont do it if you need your steem back next week, next month.