TTR, After having ridden the 520's for 2-3 seasons, I can assure you, going to the SX400 was no disapointment for me. The tall gearing can be dealt with 13/52 sprockets, and the jetting mods gave the 400 all the snot I need. It wont pull the front wheel in 5th and 6th like the 520's, but anything before that, I believe It'll hang.
Question is,... to make an SX into a woods bike, or to make the EXC a more competetive race bike. I'm a big guy, so I chose to start with the SX's stiffer suspension, (48mm forks vs 43mm) and soften the dampening a bit for woods stuff. The SX will start off with more power than the EXC due to the flywheel, silencer, and less weight,.. and there is more to be had through jetting, ect. I'm sure the same power can be had with the EXC by swapping exhaust, jetting, and installing an SX flywheel. Not sure if the SX flywheel will work the stator though. But you can always have your EXC flywheel ground to SX weight without sacrificing the E-start and lights. I believe the cam timing is the same, so no sacrifice there.
OBTW, the power gains are relatively cheap;
Silencer- $240
Jetting - EMN or DMN needle, 170 main, $10
1~ sec a/p timing - $free
Flywheel groung - $100