You'll save a lot of weight by having a much lighter wallet to carry around!!!!

If you can take a 51cm bike, then this might not be too bad of a deal.
Jamis Xenith Comp Bike 2011 > Complete Bikes > Road Bikes | Jenson USA

However, don't expect the new CF bike to make you magically able to do faster/longer rides. You need more saddle time.

Get out and try a 100 mile ride. Allow the speed to drop a little... and just ride. Often if you have a goal, you can find a bit of extra energy to get to the finish line. Make sure you have plenty of water and some calories for the longer rides (that extra 33 oz water bottle may weigh as much as the difference in weight of the two bikes).

Do you bike commute? It is a good way to get a bit of extra saddle time, and the miles add up (also a good reason to keep around the older aluminum bike).