I don't know what a streamliner could get 2-up, but in my log there are a lot of mostly 2-up tanks, especially from 2010, but my 2011 entries are even better because they contain an estimated percentage of 2-up rides per tank.
On a stock (no streamlining, stock gearing) bike it seems not to impact FE too heavily - the difference looks <10% to me. When it comes to streamlining it becomes tricky, I think - low&slow's tail makes the bike a one seater, so if you want to ride 2-up, you'll have to design a different one.
L&S: a big YAY to having to downshift
That means good in terms of efficiency, IMHO - now you have a real cruising gear and don't have to mess with high speed pulse&glide