I have added new drives and wan´t to balance all drives so that all drives have the same amount of data. 

yes i now unraid should handle this BUT

when files in a folder spread over disks and on multible accses it should be faster.

I know unbalance but there i can only spread data from one disc to all other so the 1 disc were empty 

AFAIK there's currently no way of doing that, you can set your shares to most free and in time you'll have all the drives with the same data, assuming they are all the same size, but note that writing speed will be significantly lower due to parity drive overlap because unRAID keeps switching the disk it's writing to, and really don't see much benefit for later reads, as you have no way of choosing which files go where, and can still end up reading multiple files on the same disk, though chances are lower.