Pushing in a hurry a nearly million ton ship to high relativistic speeds or even low FTL speeds with impulse engines to Delta Vega
I'd rather argue that it would be a question of pushing the ship from
the relativistic speed she already possessed, to her virtual-standstill meeting with Delta Vega. The effort would then detract from the journey home, unless it resulted in the ship regaining her near-infinite sources of power; not making the effort would allow the ship to continue towards civilization, as opposed to being "stranded".
As for dialogue cues, the ship is first said to be heading home on impulse power, but then Mitchell warns Kelso not to "activate" the starboard impulse packs. This sort of suggests coasting after an initial boost...