RAM Upgrade questions

I currently own a mid-2011 iMac which currently has 4 GB of RAM. The computer is currently running OS X Mavericks but I am thinking of upgrading it to Yosemite soon...

I often use the computer to develop iPhone apps and browse the web (often with many, many tabs open simultaneously).

When I'm there only user logged in and I only have Xcode and Safari open, xcode seems to be quite slow. What I mean by this is when I switch from one file to another it usually takes a few seconds. Even the iOS simulator seems slow.

So my question is, if I upgrade the ram to 8 or 16 GB, will it help to fix the slowness?

Also, if I upgrade to Yosemite, will that affect the speed of the computer?

Thanks in advance