Critical Mode will be a lot easier since you know what you're doing now and know how melding works and whatnot.
When I first played, I completed all three stories on Proud Mode, and lost on almost every boss with Ven and Terra at least once. Terra's was the most difficult for me. I just completed Critical Mode with Terra, and it was extremely easy. I didn't use Shot Locks, D-Links, or even commands for most bosses (except Curaga) since it made the battle too easy. Even the
battle and the final battle only took me a few tries with the above mentioned restrictions. I even had to equip Zero EXP from time to time in fear of overleveling, and completed the game at Level 28. However the secret bosses are still a pain for me to defeat.
I'm considering doing Level 1 CM playthroughs for both Ven and Aqua.
I gotta admit I'm slightly disappointed at how simply "knowing" the game makes it so much easier. The difference in difficulty is no where near KH2 Proud vs. KH2FMCM.
Admittedly, having "Second Chance" and "Once More" this time around is one aspect that slashed down the difficulty, but it's still not the only factor.