I was thinking about doing the same thing. I found info show the late 1K wheels at 10.25 and 16.75. The rear Busa wheels shows 19.5. So 2.75 rear"¦ I'm guessing the front will save you 1.5, maybe 2 lbs max.
You'll need custom spacers for the 1K wheels as well as the 1K rotors, cush drive, sprocket, and cut .250 off the busa brake hanger. I've also read both yes and no that a custom front axle is required (May depend on the year) Total loss of about 4-5lbs.
Most say use the 05+ 1K wheels. They are a bit lighter. Numbers above are based on 05+ wheels.
I decided the weight loss wasn't significant enough unless you had a heck of a deal going on the 1k wheels.
A set of Marvic Piumas will save you about 8 lbs on the rear for a total of about 13lbs.
BSTs will save another 3lbs.
If you decide to go the 1K rout, yell and I'll point you toward a couple of guys making conversion kits for 1k wheels. They should be able to tell you the fitment details.