Borg transwarp is similar to Quantum slipstream according to seven of nine. I'm pretty sure with all the data Voyager have on transwarp and QS that starfleet R&D could have some sort of working system with a year or so of Voyager's return.
They already have a fully functional QS drive v2.0 (10 000 Ly's in 1 minute).
SF merely needs to resolve the phase variance and that's it, or for starters, they can use the v1 that Voyager originally procured from Arturis ... 15 Ly's in 15 seconds if real time on the show is to be taken into account).
That's 60 Ly's in 60 seconds, or 3600Ly's in 1 hour (beats the heck out of fastest warp speed.
Of course, the v1 of QS meant that Voyager would need to be in it for 3 months before reaching Earth (60 000 Ly's away at the time).
That's about 714.285 Ly' per day (or 5000 Ly's in 1 week) - significantly slower than 1 Ly per second as was implied before the fake Admiral Hayes messed it all up.
However that STILL beats any fastest Warp speed.
That v1 had problems in terms that the quantum stresses would tear the ship apart after 1 hour.
Still... that's 29.75 Ly's in 1 hour if we stick to the 3 months to Earth in QS (while in the show, the ship got 300 Ly's in 1 hour after already spending how much time chasing Arturis to bring Janeway and 7 back - which was significantly faster - lol - the writers don't know basic math?).
Either way, the v1 of QS wheter it takes you 3 months to cross 60 000 ly's or just over a week is essentially much faster than anything SF was able to achieve with stable warp speeds (strange accidents or alien influenced machines not-withstanding). And seeing how v1 had quantum stresses to deal with, I would imagine that a SF can find a way to lower those stresses so they can either increase the time under QS, or simply negate them all-together.
Either way, they can get v1 operational faster than v2 probably, and v2 wouldn't be far behind (or might be for a few years if their computers are simply too slow to compensate for the phase variance occurring in the Slipstream envelope - though it wouldn't be impossible to fix even with their tech).