Wow...nice bike!!! Personally, I'd keep your current bike if you like it. Do you mostly do tris or TTs? I know of many very fast guys here in SoCal on older TT bikes that MIGHT gain a few seconds with a newer super bike. I haven't seen data, but folks on the Slowtwitch forums say they've seen wind tunnel results that put the Slice RS on par or better than the Shiv TT or Speed Concept.
If money isn't an issue and you don't mind tinkering, the Slice RS is pretty cool. The ride and handling are awesome. I needed a new bike anyway when I got mine, since my old Felt B2R had a cracked frame. I also have a Chinese TT frame for training. The Slice RS is definitely unique in most areas populated with Cervelo and Specialized bikes. I just did a TT a few weeks ago in strong winds on a curvy downhill (return uphill course) and finished ahead of several riders who normally beat me. I think my Slice RS played a big role in this.
Again, if you're super fit and are primarily focused on training, you'll probably be fastest on a Cervelo P3 with a well fit and trained position. The good news is that your Cinelli has many of the features of the P3 (like many other companies have copied). Of course, so much of what we do in TTs comes down to training #1, then position, our clothing, helmet, and wheel selection.
Good luck and you can't go wrong with either your current bike or getting a Slice RS...