Funny you should post this now, we were just discussing something very similar over on this thread!
http://www.trekbbs.com/threads/star...isruption-technology-alteration.277476/page-5
The biggest problem in importing TOS speeds into the TNG era isn't TNG (which often flew vast distances, especially in its early seasons) or DS9 (which at times seemed absurdly close to Earth, from the travel times) but that dratted USS Voyager and it's supposed top cruising speed of Warp 9.975, along with the stated return trip of 70,000 ly in 75 years. However, what if the problem is the starship herself?
Hailed from the start as a new and high-tech starship, Voyager probably sported more types of experimental technology than just the bio-neural gel packs (and which almost crippled the ship when they caught space-flu). Specifically, I'm thinking of those teeny-tiny nacelles, almost half the proportional size of the ones on the Enterprise-D. Never designed as a long range explorer, the Intrepid class may have been the testbed for a new type of nacelle - smaller but more technologically advanced, able to fully utilise the power output of a super sized "tower" warp core thanks to special <tech> components. Unfortunately, when these were burned out by the Caretaker's displacement wave (both had incompatible energy emissions) the diminutive nacelles were reduced to a fraction of their former propulsive power. Like the gel packs the <tech> components could not be repaired when damaged, and spares were only available from properly equipped Starfleet bases - not normally an issue in Voyager's assigned patrol area, but a massive problem when on the other side of the galaxy! The 9.975 Warp Core remained intact, but the warp coils themselves could only handle such power outputs for limited periods of time and even then at much reduced efficiency compared to their more traditional equivalents (see quoted sprinting speeds in
Manoeuvres and
The 37s).
Incidentally, we can see that the "small but advanced" type of nacelle never really caught on - the more robust and traditional type (big = fast) is still in effect on board the Enterprise-E
A final point - Voyager herself only averaged about 400ly a year for most of the series - the rest of her progress was made using helping hands and shortcuts. This really wasn't a speedy ship!