To add to what Timo and others have said, especially regarding the lower-power but easier-to-govern warp core, I am inclined to think that the Nova-class was designed to be able to sustain Warp 8 indefinitely, or at least as long as is needed to get where it's going. Seems like most Trek ships are described on websites and in tech literature as having three rated speeds:cruise, maximum, and 12-hour dash, and it appears that cruise is probably for best efficiency and fuel consumption, while maximum is sustainable for as long as they wish without causing damage or problems. Larger capital ships seem to be in the ranges of Warp 6 cruise, Warp 9 maximum, and Warp 9-point-whatever for dash. That being the case, a relatively small ship like the Nova-class sustaining Warp 8 indefinitely is pretty respectable, gets them to where they need to be within an acceptable time frame, and frees up the in-depth exploratory missions so the larger capital ships can go about their business. Clearly, the "Equinox" plot required the Nova to have a reason to be slower than an Intrepid, but the Warp 8 maximum speed still fits acceptably well with in-universe explanations above.