Hey Roger & CrazyChem, thanks for the helpful info.
Roger: I'm 140 pounds and 5'11" tall. I'm fit and strong for my weight. I have been told by a few sailors that given my weight, I could get away with a larger sail than is recommended on the board. I have never tried it, so I'm not sure this is true.
Yup, the Rumba has a centerboard. The only time I have it down is when I'm uphauling. I've tried sailing with it up and down (never down when jibing) but I haven't been able to notice any difference, so I just sail with it up. I'll try a lighter mast with the 6.4 (yes its the original rig). I tried using the 490 carbon mast that came with my 8.5, but it was too stiff.
One of my concerns about going with a 120-130L board is that I'll outgrow it quickly. On the other hand, I'm not sure I'm ready to handle anything less than 120L.