...but from looking at the geometries and tube lengths, the 56 Cannondale will be v. similar to my 56cm Trek, but with a lower headtube (which is what I want). However, I think I could get myself easily onto a 54 if I used a longer stem and laid my seat back slightly more...
I can't tell you if the 54cm Cannondale will fit you, but do not go moving your saddle around to correct your reach on the bike. As a general rule you need to set saddle height and setback first. These measurements should be based on your relationship to the bottom bracket. Then worry about where your hands fall. The difference between a 54 and 56 TT lengths can be made up with a 20mm stem increase, i.e. 100mm to 120mm. The stack height can be changed with spacers and stem angle. If a couple of CM's of spacers don't get your hands high enough you need to stick to the larger frame size.
The smaller frame will probably be slightly lighter, but discomfort affects your ride far more than a few extra grams ever will. Get the size right first, then try to save weight.