The only consistency about warp speed is its inconsistency--which could be explained if warp factors vary depending on local subspace conditions (Warp 4 here
could be faster than Warp 9 there
). And some ships may be able to traverse vast distances of the Galaxy very quickly if they know the location of "warp highways" (or subspace shortcuts). A Federation starship stuck in an unfamiliar part of the Galaxy may not, IMO.