Personal opinion, if you like your liter bike, keep it and buy and ST1300 for 2 up. Your passenger will like you much more than on a busa. Nothing wrong with a busa if you feel the busa is the right bike for you. However if you are looking for it as a good "hauler", there are much better candidates available.
Listen to this advice!
I went from an 03 Busa to the K5 GSXR1000 and back on an 07 Busa, and if I could afford it I would have a 1000 and a Busa and a FJR1300 or Concours14
The 1000 would crush the Busa from a 60mph Roll on in 6th gear
for example........
I picked 2 road signs to test my roll on's between...
K5 GSXR1000 50MPH to 90MPH between the signs....
03 & 07 Hayabusa 50MPH to 78MPH between the signs....
NO CONTEST.......
Now on the Dragstrip....
My K5 GSXR1000 My best of 11.00 at 136mph with a 1.94 60foot time.
My 07 Busa.....my best of 10.93 at 134MPH with a 1.97 60foot time.