1) IIRC, you lose about 10% of LF.
2) theoretically, yes.
However, from my own experience:
- I rarely do sleepovers, rather overnight flights, and I don't care if they depart at 2355 or land at 0500 (always better though, if you can avoid it)
- When I do, I land the latest I can and depart the soonest possible. Wasting 2 more hours is not worth it, when you compare it to the immobilization of your plane for 2 hours (idle=costing money).