There's truth to Luke's reply about the gyroscopic effect of bigger disks. However, since the RS250 runs 296mm disks, I'm inclined to believe that the bigger disks are a better option. I've raced the SP on SP disks, and also on RS disks. Give me the 296mm RS stoppers anytime.
Luke, are you sure about the weight of the SP v R disks. I'm pretty sure that the R spec are painted steel inners, while the SP are anodized alloy. Hmmm, needs checking methinks.
Floating the SP disks is a good option. Gives a better feel in my opinion and less chance of any judder if the disk isn't perfectly true. The flash option is to do what Luke did and fit the Brembo buttons, or there's a cheapo option where you can press the stock buttons to make them float. I've done this plenty of times, not just on NSR and it works just the same. In fact it was essential on the old R6's as when they shook their head they threw the pads off the disk so you had no brakes. Pressing the buttons reduced this, which was a good thing.