My impression is that now that the centre seal is available for the R2J and R4J crank, that seal is more durable than the integrated seal in the 5K and later cranks, so that might be the better option if I am going to re-build the crank anyway. Is that right?

How about the electronics? Does the PGM have to match the crank (I read the placement of the pulse generators is different)? If so, is the PGM II any better than the PGM I for a racer/track bike?