Maybe, maybe not on both of your statements.
One venue I work in has the PA flown fairly far upstage, right around the plane of the kick drum. It's a fairly big room (1000 cap.) and a good line array with good (predictable and smooth) coverage pattern. This means the PA is maybe 20' behind the front line vocal mics. I have no problem with feedback, the stage sounds really full from some inclusiveness of the FOH, I have plenty (really plenty) of volume and those in the front rows (high paying seats) do not have PA in their faces and there's no sightline issues. These are the $50-100 seats, so by having the PA where it is means that on some shows, those seats pay for the PA and the lighting costs. Pretty good trade-off in most cases. Some of you here might also recognize the POTENTIAL improvement in the dound of the back line due to POSSIBLY improved time alignment. The reason I hedge on this is that it varies with each point in the room, so on the average it may be a little better but it's not absolute.
How far apart? Depends entirely on the band and stage/room situation. I do not like to get too far apart unless I am flying upstage and the room is reasonably narrow. Otherwise, the sound no longer comes from where most folks feel the sound should be coming from. Localization is one of the parameters in situating a speaker system relative to the stage and audience.
