I have a set of Nissin calipers, braided brake lines and better fluid setting in my garage, I will probably put them on next weekend. Stock are good but Nissin, lines and better fluid are alot better in my book.
I've got 310mm discs from a ZX14, the same size as gen 3, and I've tried ZX14 Nissins and 08 Gen 3 Tokicos. The Nissins are much stronger, I had oem pads in both sets, and I felt they were a bit too much for the road on my Gen 2.
I think you'll need spacers if you use the Nissins, I'm pretty sure the Tokicos have an extra 5mm-ish built into the caliper.