In the most recent episode we learned, via Discovery making the trip from Jupiter to Earth in roughly ten seconds with the warp core powered down and all it's Dilithium hidden on another ship, that Discovery's fusion engines can get the ship traveling somewhere around 262 times light speed.
Which according to the official scale puts it somewhere around Warp 6 on the TOS scale, and somewhere around Warp 5 on the TNG scale.
This means that with just Discovery's level of Warp technology, Discovery's level of Fusion technology, and Discovery's reactor size to internal volume, warp travel should have still been viable, if limited to slightly above/around Enterprise era speeds.
But before people say that still might not be enough, look at the three qualifiers. Because 1) We know from DS9 that the Federation has improved it's fusion technology, 2) we know from VOY that the Federation has improved the warp geometry and field formation of its ships, 3) we know from Enterprise that they can make ships which are bigger on the inside, and 4) we know from discovery itself that's there's a lot of extra space they could install additional fusion reactors if they needed to.
So why do they need Dilithium?
Which according to the official scale puts it somewhere around Warp 6 on the TOS scale, and somewhere around Warp 5 on the TNG scale.
This means that with just Discovery's level of Warp technology, Discovery's level of Fusion technology, and Discovery's reactor size to internal volume, warp travel should have still been viable, if limited to slightly above/around Enterprise era speeds.
But before people say that still might not be enough, look at the three qualifiers. Because 1) We know from DS9 that the Federation has improved it's fusion technology, 2) we know from VOY that the Federation has improved the warp geometry and field formation of its ships, 3) we know from Enterprise that they can make ships which are bigger on the inside, and 4) we know from discovery itself that's there's a lot of extra space they could install additional fusion reactors if they needed to.
So why do they need Dilithium?