Actually, Voyager's Warp 9.975 was considerably slower that the Classic Enterprise's warp 8.4. The former crosses the galaxy in 75 years, the latter, a month. Relavent clips in the video in my sig....
The TOS conveniently utilized speeds that are WAY beyond late TNG, DS9 and VOY shows - this is because the writers felt the Galaxy is becoming 'too small', hence they lowered the speed.
In the early TNG (seasons 1 and 2 I think) the Enterprise-D for example would need roughly 2 years and 7 months to reach the nearest Starbase at maximum speed (sustainable Warp 9.2 without breaks if I'm not mistaken) which was roughly 7000 ly's away.
At those speeds alone, Voyager would need roughly 30 years to get back to the Alpha Quadrant.
But since Voyager's maximum sustainable cruising speed is 9.975 (and since the speeds experience exponential increase with small factors past 9.9 the closer you reach the threshold), Voyager would probably require anywhere between few to 15 years to get back.
Point being that there was a revision of the Warp scale for TNG.
I don't have anything against ludicrous speeds in TOS and then of course in TNG - it would be in line with Federation capabilities and a notion of humanity being an 'emergent' society where by the 24th century they would be exploring other galaxies (and not just the Milky Way).
As for Prometheus... from a Tactical Point of view it can definitely see certain benefits.
I think that SF merely expanded on the separation capability of Enterprise-D by providing the Prometheus separated sections more firepower and independent Warp capability (whereas the saucer section for all intent and purposes could merely 'sustain' a certain Warp velocity if the separation occurred at Warp speeds in the first place) - it was designed for deep space tactical assignments after all - so if it can separate, it stands to reason that you should beef up the sections.