Cannondales's are big fun to upgrade because almost everything imaginable is available and will fit the bike....no proprietary issues!
We've had 3 C'dale Tandems. Two were first generation frames, one of which we had custom built in '90 and then extensively upgraded in '07. I put every carbon piece, Dura-ace, and XTR goodie on it that I could afford but I could never find a proper 1 inch carbon fork for it. Never the less, all of the upgrades made a good old Tandem an even better current Tandem.... we loved that bike and never regretted spending waaaay too much money on all the upgrades, ...it was a great ride.

Since '09 we've had a couple of other Tandems with the most recent being a 2010 C'dale Road Tandem. Straight out of the box this current generation Cannondale is a super bike and a great value as repeatedly mentioned on this forum. One of the things I really appreciate about this C'dale is the very positive feel that the huge Fatty fork gives the front of the bike particularly on steep, twisty downhills and under hard braking. I'm not so sure I'd be quick to risk going to a carbon fork for the kind of riding that we do. The only upgrade I would be tempted to go after, if I did not love the disc brakes so muce, would be to try some high quality go fast wheels along with a change to caliper brakes. I think you are right on target with the wheels and believe that there is speed to be found in that area particularly in the acceleration phase.
What are you doing for brakes with the Rolf's?
Bill J.