My setup up until recently was standard size 300mm RPB discs, DS2500 pads, braided lines and a brake stopper. All very good, much improved over the OEM stuff and easily good enough for a little track time.
Recently changed to the K-System 325mm kit. Overall power is obviously up but the biggest improvement is the feel\modulation of the brakes. Another advantage of the 350Z discs is further scope for upgrade, plenty of choices for 2-piece units if you want of need them. Probably the cheapest kit out there too, but doesn't fit under standard wheels without spacers.
Unless you're doing extended track sessions or really need the bling factor a BBK is not really needed, upgrading the factory spec components is sufficient. As Chris said service what you have first, makes a big difference.