On how many House seats Dems would gain:
Might as well be the contrarian. -4.
Not impossible, nor unlikely. Last time we were in a similar situation with Bush, the Rs picked up a few seats in the midterms. Right now RCP has the race as essentially tied.
Remember, this isn't a presidential election year and Rs tend to do better in the midterms.
If the Senate is going to have R pickups then we're likely to see a similar situation in the House.
I've seen polls showing they are even in the generic ballot which means that Rs will net a bunch of seats.