We do have a few instances from TNG stating that Warp speed of 9.2 and 9.4 are much faster than what was stated for Voyager.
Also, even Paris himself mentioned in 'The 37's' episode that Warp 9.9 is 'about 4 billion miles per second', which comes out to 21,473 times the speed of light.
From TNG in Season 1, 'Where no one has gone before' episode, Data stated that the Enterprise-D would take over 300 years to traverse 2.7 million Ly's at maximum Warp.
From 'Encounter at Farpoint', we learned that the maximum Warp velocity (which puts strain on the engines) the Enterprise-D can achieve is Warp 9.2 (though they were able to match Q's net which was travelling at warp 9.8 'at an extreme risk' - obviously they were unable to sustain 9.8 for very long, so 9.2 was at the time the maximum [cruising?] limit).
In 'Q Who', the Q has thrown the Enterprise-D 7000 Ly's from their previous coordinates.
Data estimated that at maximum Warp velocity, the ship would need 2 years and 7 months to reach the nearest starbase (which granted we don't know how far away it was from their relative position, BUT if we estimate that the Enterprise was likely on the (or close to the) border of Federation space to begin with, then it is possible the starbase in question was about 6000 to 7000 LY's away).
So, Warp 9.2 from Season 1 would equate to roughly 41.2 Ly's per day (or 15330 times the speed of light - with a reasonable estimation that crossing 2.7 million Ly's would last about 305 years).
In 'Q Who', The Enterprise-D was able to achieve Warp 9.8 easily enough on Picard's orders, whereas 9.9 was established as their 'maximum'.
So, if Warp 9.9 from ‘Q Who’ takes 2 years and 7 months to traverse 7000 lightyears, that puts it at roughly 2800 times the speed of light (if I'm not mistaken) - a much greater reduction in speed from Season 1, which continued to reduce until it landed at 9.975 = about 1000 ly's per year for Voyager (and Ds9 also mentioned this – but even Ds9 was inconsistent at times with its Warp speeds).
Now... I personally find Paris statement about Warp 9.9 being 4 billion miles per second much more in the range of what is possible for the Federation in the late 24th century by the time Voyager was launched and a decent/consistent increase over Warp 9.2 from Enterprise-D.
I think it was stated that exponential increase doesn't happen until you pass Warp 9.9, meaning that with every increment past 9.9, the velocity of the ship doubles.
So, Warp 9.975 would equate roughly 11452.2 Ly’s per day (if we go by Paris statement of 4 billion miles per second) - and this would allow Voyager to get back to the Federation in just under a week.
Incidentally, this speed would mean the ship would traverse 440.46 Ly's every hour (which is 140.46 Ly's faster than Slipstream version 1 maximum velocity of 300 Ly's per hour which the fake Dauntless used).
If we go by the speeds from ‘Q Who’ though, Warp 9.9 = about 7.67 Ly’s per day … and 9.97 would be roughly 981.76 Ly’s per day, and Warp 9.975 would be about 1472.64 Ly’s per day (537 513 times the speed of light) – at this velocity, Voyager would take just under 51 days to traverse 75 000 Ly’s.
As for Voyager taking 75 years to get back to Federation space... some people have stated that Voyager used Kirk era Warp scale instead of the revised one from TNG which put Warp 9.975 at those low speeds.
What I find more credible as to why it would have taken them that long, is that the Caretaker's pull damaged the ship on a level the crew was unable to repair while so far away from home, resulting in much lower attainable Warp velocities (the ship WAS indeed critically damaged after it was pulled to the DQ).
We even saw that Voyager was unable to sustain high Warp in 'Threshold' episode and the computer said 'nearing maximum warp velocity' [which in this instance would be 9.975 according to the dialogue].
But, voyager was repeatedly stated on the show that Warp 9.975 is its 'top cruising speed' or its 'maximum sustainable cruise velocity'.
Maximum cruise velocity indicates that an engine would effectively be able to sustain it indefinitely without trouble... but apparently, this did not carry out.
Which leads me to think that Voyager's hull or Warp systems (or both) took heavier damage in transit to the Delta Quadrant which they were unable to isolate or repair in full, forcing the crew to use lower Warp velocities at all times and lowering their top cruising speed as a result – and it was never mentioned.
It wouldn't be the first time that characters said something inconsistent or failed to describe completely.
Various individuals kept asking themselves how was Voyager able to destroy various shuttles and always had more to spare… or its torpedo compliment being initially 36 I think which Janeway said there was no way to replace after they are gone.
In short, I think the writers wanted to make things more dramatic while ignoring their technical capabilities.
For instance, Voyager could easily replicate various torpedo parts and casing and just assemble the thing without antimatter. Then trade with other friendly species such as the Talaxians or others who had antimatter to spare so they can supply their photon torpedoes as opposed to replicating antimatter (or in emergencies, the crew might have used antimatter from the warp core itself).
Similarly, Voyager made numerous pit stops in uninhabited star systems and harvested raw materials which they probably used for easier replication – matter to matter (as opposed to using direct energy to matter process) that resulted in reconstructed shuttlecrafts.
Damaged shuttlecrafts were likely recovered off-screen and then salvaged as much as possible.
We’ve seen them building the Delta Flyer after all, so its not beyond their capabilities – albeit I found a distinct lack of automation for Delta Flyer construction a tad unrealistic.
