Would there be a noticeable difference...
 

Between an inner barrel which is 395mm vs a inner barrel which is 273mm? In terms of accuracy, fps, range, etc?

depends.....
for which gun
for what purpose

y'know specifics

as it stands I'll say
definitely maybe

On PTWs some people claim to have better accuracy with the shorter barrels
Not 100% sure on AEG's since I haven't done a whack of testing, but I can tell you the difference will be nothing more than slight.
More critical than barrel length to range and accuracy would be your ammo weight and hop rubber

It really depends on the barrel. My VFC CQBR is way more accurate than my ICS AK74M with the same hopup rubber and BBs.

I'm trying to decide whether I should get the kwa erg scout or wait for the shorter cqb version. I don't want to lose out on accuracy thou. The plan is to Rhop

Most of the time it's how straight the barrel is. Slight variances in the barrel will pooch your accuracy. Any quality after market inner barrel should provide significant improvements to accuracy on this alone not just length, if the length is the same as stock you wont need to mess with your hop up either

Hmmm, interesting stuff.

The CQB gun has an FPS of 330 according to KWA's website while the scout has an fps of closer to 400. I could just drop a stronger spring in to get it up to the 400 fps range or do I need to start worrying about cylinder pressure and a shorter barrel?

The gun should come with a properly ported cylinder for that length of barrel. Dropping a higher FPS spring in will be no problem.

If you're getting it in Canada, though, chances are it's gonna be closer to 400fps anyway, despite what KWA's website says.

So really I should just wait for the CQB one. Shorter gun, easier to maneuver with and same performance?

That's a personal preference... you gotta decide that one yourself :p