Can I move System/User folders to separate disks?
Haven't tried all of the things you are asking, but I have succeeded in moving /users to another partition.  Mount it as /Volumes/Users, and make sure there is symbolic link from /Users to /Volumes/Users. 

There is a bit of a dance:  Login as root.  Sudo doesn't work for this.
mv /Users /UsersOld
mount newdisk /Users
copy /UsersOld/* /Users
for each user: 
   chown -R his directory.

You may be chasing down ACL issues for a while.  

You may be able to do the copy with tar.
e.g. cd /UsersOld; tar -cf - . | tar -C /Users -xvf -

This will do the right thing for permissions, but I don't recall if it does the right thing for resource forks and ACLs.

You may need to mount it at a different point, creating an appropriate fstab file with an entry for it.

I would suggest that you enable root as a login account so you can have a GUI to snurfle around when things go pear shaped.

As to the efficacy of what you are doing:  It won't make much difference.  The read/write performance of having stuff at the edge vs spread out isn't much.  You're talking at best 20% or so, and more likely far less.

If you have not done so, max out the ram your laptop can hold.  This does far more for increasing performance than any other upgrade you can do, especially if you are using data pigs like photoshop.