The problem with the saucer as a lifeboat is of course that episodes contradict each other. In "Brothers" saucer separation is considered an option not just because separating will let Our Heroes regain control of the ship (well, a good chunk of it), but because the saucer -will- drop out of warp afterward.
Yet as Timo notes, based on EaF the saucer can travel much faster than impulse when separated.
It doesn't resolve the "Brothers" situation at all, but perhaps the best compromise is that the saucer has a "warp sustainer" engine similar to how photon torpedoes work. Fire at warp and they stay at warp, but they can't get to warp by themselves.