Might depend what you mean by `generic`. Jagwire cable is stock on some new bikes. And there are several grades of Jagwire cables. They also make cables for Shimano so you may just be replacing a brand name.

And like many other bicycle components - proper fitting and installation is hugely important. Optimal performance is determined by the cable run itself as well as the proper finishing of cable ends to minimize friction and the selection of ferrules appropriate for the job.

You`d be most likely to FEEL a difference on installations that use higher end shifters because they require less effort to operate in the first place. However, more accurate indexing (and therefore shifting) would be a benifit in just about every shifter. Accurate indexing becomes increasingly significant as the number of gears goes up.