Memory question

One simples question.

Imagine I have 300 unique objects, each one with a big list of behaviors.

This takes away memory somehow? or it would make the game faster by ading all that behaviors to the event editor?

(I am using flash, so this is a big concern)

It won't make any difference.
The main two things that will cause slow-down are fastloops, and displaying graphics (esp. with any kind of semi-transparency / effects).

I've been doing optimizations for my flash game recently, and large actives with an alpha channel will definitely cause considerable slowdown, I did notice minor improvements when limiting the number of conditions I was using.

Other than the big 2 that muddymole mentioned, here's my experience:

Things I found that helped performance were placing code into groups and deactivating them when not needed, and also giving objects group qualifiers, and checking against the group instead of individual objects.
Also, having a large amount of objects on screen with calculations (especially collision checks) will cause slowdown too.
These things could cause minor slowdown, but nothing major.

Also, if you want to see your game at it's worst, run it in internet explorer.
My current project (set at 50fps) at times will dip to 46-47 fps in Chrome during certain events, but in IE it will dip down as low as 35 fps during the same event.

Internet Explorer is an appalling piece of junk though, and the poor performance isn't even the worst part.

You should try actually designing a website - practically everything seems to require some kind of work-around to function in IE.
For example, the latest thing I discovered, was that you can't have an element with the same name as a javascript variable. Any other browser, it works perfectly, but not IE :(
Opera is still the best, but Chrome is definitely getting closer...