I'd say the fact that the saucer wasn't able to escape Veridiian III in "Generations" proves it well enough.
That bit of ST:GEN proves the saucer can't move at impulse, either...
Or then the only thing it proves is that big warp cores exploding next to you will generally result in a substandard performance, or sublight performance if you will.
If ship separation had been written to more episodes, it would probably have been dramatically useful to give the saucer relatively good FTL capabilities. That way, the heroes could have two starship adventures with one starship at the same time, without the need to write one of those adventures with atypical and cumbersome limitations...
Timo Saloniemi