Well, If the 54cm-650c fits well and you're looking for a 700c equivalent in the P3 SL, that would be the 53cm P3 SL. They are very close in reach, only 6mm different and if you take into account the small difference in stack (also 6mm), the actual cockpit length difference between the two bikes is 7mm, easily adjusted through a slightly different stem.
Gerard Vroomen
3T.bike
OPEN cycle