Reach-wise, yes, but not stack-wise. The L Transition actually has ~1cm greater stack than the 56 P3C (which accounts for ~0.3cm of the shorter reach.)
Besides, the XL was changed for 2010 and now has a 2cm higher stack, as opposed to the previous years where the stacks on the L and XL sizes only differed by 1mm.
I also find it sort of interesting that you'd ride those 2 sizes...since, IIRC, I'm at least 3" shorter than you, and a 56 P3C is what I had borrowed for the "Something Borrowed..." testing (and fit great) and I'm thinking about getting a L Transition. Of course, the fact that I'm now running a Scott 100K bar probably has some influence on that (i.e. it requires a shorter stem for a given pad position than the Vision bars I had used on the P3C).
In any case, you lost the point I was trying to make, which wasn't how the P3C compared to the Transition, but how the P3C compared (by inference) to the Shiv...and how that didn't match Gary's summary above.
http://bikeblather.blogspot.com/