The next step is brakes. It's easy, it's cheap, you can do it yourself, and it will make you feel like Superman, ESPECIALLY after doing the front springs and adjusting your suspension. After I did it, I was kicking myself in the head, trying to figure out how (and why) I ever rode it before.
The following tutorial was written on the Gen I Hayabusa brakes:
http://www.suzukihayabusa.org/forum/index.php?topic=75357.0Your bike is a Gen II, so the part numbers will be different.
The correct part number for the EBC HH sintered front brake pads for the Gen II is FA379HH.
The "kit" race version of the above pads are part number GFA379HH. The "kit" race pads work about the same as the regular HH pads below 150 mph, and feel stronger than the regular HH pads above 150 mph.
The EPFA379HH is their "Extreme Pro" version of the "kit" race HH pads, and if I understand it correctly, it's even more powerful than the regular "kit" race pads.
The GPFA379HH is even more powerful than that, the MOST powerful "kit" race pads that they make, and they caution about using them on the street. These would be the ones that I would use on a turbo Hayabusa with good suspension and tires.
A word of caution- these pads will give you that "Hand of God" stopping power, like a jet fighter catching a 3-wire on a carrier deck. You'll need to take time to become accustomed to having that much power available. The kind of braking power we're talking about here is the ability to lift the rear tire off the ground at ANY speed, using just your pinkie.
Their authoritative initial bite is really confidence inspiring when you're going really fast, and for as powerful as they are, they are also very linear and predictable. They also give great feedback, including audible feedback, they are very fade-resistant, the last a long time, they're inexpensive, they're EASIER on the rotors than even the stock brake pads, and they leave virtually NO brake dust on the wheels. They're just plain awesome.