The clouds seen in FS2004 aren't volumetric clouds, those are 2D layers, crossing each others in the distance and pointing in the direction of the airplane ... that's why FS2004 can keep up the speed and display the cloud system in realtime.
But they did a great job with those clouds, because they look close to reality and this system allows for lots of different cloud versions and weather situations.

Volumetric clouds will be used in the upcoming IL2/Pacifc Fighters follow up "Battle for Britain".

But you're right, the online weather link works pretty good - not realtime - it calls up stored info of weather situations from about 15 min. to 1 hour ago.
It is a cool function and working very well.